[BUG][MAVEN] Classpath $ref to external file are not resolved correctly

#### Bug Report Checklist

- [ ] Have you provided a full/minimal spec to reproduce the issue?
- [x] Have you validated the input using an OpenAPI validator ([example](https://apidevtools.org/swagger-parser/online/))?
- [x] Have you [tested with the latest master](https://github.com/OpenAPITools/openapi-generator/wiki/FAQ#how-to-test-with-the-latest-master-of-openapi-generator) to confirm the issue still exists?
- [x] Have you searched for related issues/PRs?
- [x] What's the actual output vs expected output?
- [ ] [Optional] Sponsorship to speed up the bug fix or feature request ([example](https://github.com/OpenAPITools/openapi-generator/issues/6178))



##### Description
#18576 PR for openapi-generator-maven-plugin inputSpec -- Allow jar: URLs introduced support for rewriting `inputSpec` in openapi-generator-maven-plugin but it broke references to external files.

We have a similar maven plugin configuration:

```xml
 <plugin>
        <groupId>org.openapitools</groupId>
        <artifactId>openapi-generator-maven-plugin</artifactId>

        <dependencies>
          <dependency>
            <groupId>com.example</groupId>
            <artifactId>your-artificat-with-openapi</artifactId>
            <version>your-version</version>
          </dependency>

        </dependencies>

        <executions>
          <execution>
            <id>openapi</id>
            <goals>
              <goal>generate</goal>
            </goals>
            <configuration>
              
              <inputSpec>openapi/openapi.yaml</inputSpec>
            </configuration>
          </execution>

        <executions>
 </plugin>
```
Then we have multiple OpenAPI files in artifacts (e.g. openapi.yaml, model-openapi.yaml) and use references between them.
```yaml
schema:
  $ref: "model-openapi.yaml#/components/schemas/User"
```

During the build with openapi-generator-maven-plugin it fails and throws this exception:
```
Exception while resolving:
java.lang.RuntimeException: Unable to load RELATIVE ref: ./model-openapi.yaml
    at io.swagger.v3.parser.util.RefUtils.readExternalClasspathRef (RefUtils.java:150)
    at io.swagger.v3.parser.ResolverCache.loadRef (ResolverCache.java:162)
    at io.swagger.v3.parser.processors.ExternalRefProcessor.processRefToExternalSchema (ExternalRefProcessor.java:89)
    at io.swagger.v3.parser.processors.SchemaProcessor.processReferenceSchema (SchemaProcessor.java:238)
    at io.swagger.v3.parser.processors.SchemaProcessor.processSchema (SchemaProcessor.java:59)
    at io.swagger.v3.parser.processors.ResponseProcessor.processResponse (ResponseProcessor.java:56)
    at io.swagger.v3.parser.processors.OperationProcessor.processOperation (OperationProcessor.java:86)
    at io.swagger.v3.parser.processors.PathsProcessor.processPaths (PathsProcessor.java:88)
    at io.swagger.v3.parser.OpenAPIResolver.resolve (OpenAPIResolver.java:72)
    at io.swagger.v3.parser.OpenAPIResolver.resolve (OpenAPIResolver.java:59)
    at io.swagger.v3.parser.OpenAPIV3Parser.resolve (OpenAPIV3Parser.java:240)
    at io.swagger.v3.parser.OpenAPIV3Parser.readContents (OpenAPIV3Parser.java:183)
    at io.swagger.v3.parser.OpenAPIV3Parser.readLocation (OpenAPIV3Parser.java:97)
    at io.swagger.parser.OpenAPIParser.readLocation (OpenAPIParser.java:16)
    at org.openapitools.codegen.plugin.CodeGenMojo.calculateInputSpecHash (CodeGenMojo.java:1012)
    at org.openapitools.codegen.plugin.CodeGenMojo.execute (CodeGenMojo.java:985)
    at org.apache.maven.plugin.DefaultBuildPluginManager.executeMojo (DefaultBuildPluginManager.java:126)
    at org.apache.maven.lifecycle.internal.MojoExecutor.doExecute2 (MojoExecutor.java:328)
    at org.apache.maven.lifecycle.internal.MojoExecutor.doExecute (MojoExecutor.java:316)
    at org.apache.maven.lifecycle.internal.MojoExecutor.execute (MojoExecutor.java:212)
    at org.apache.maven.lifecycle.internal.MojoExecutor.execute (MojoExecutor.java:174)
    at org.apache.maven.lifecycle.internal.MojoExecutor.access$000 (MojoExecutor.java:75)
    at org.apache.maven.lifecycle.internal.MojoExecutor$1.run (MojoExecutor.java:162)
    at org.apache.maven.plugin.DefaultMojosExecutionStrategy.execute (DefaultMojosExecutionStrategy.java:39)
    at org.apache.maven.lifecycle.internal.MojoExecutor.execute (MojoExecutor.java:159)
    at org.apache.maven.lifecycle.internal.LifecycleModuleBuilder.buildProject (LifecycleModuleBuilder.java:105)
    at org.apache.maven.lifecycle.internal.LifecycleModuleBuilder.buildProject (LifecycleModuleBuilder.java:73)
    at org.apache.maven.lifecycle.internal.builder.singlethreaded.SingleThreadedBuilder.build (SingleThreadedBuilder.java:53)
    at org.apache.maven.lifecycle.internal.LifecycleStarter.execute (LifecycleStarter.java:118)
    at org.apache.maven.DefaultMaven.doExecute (DefaultMaven.java:261)
    at org.apache.maven.DefaultMaven.doExecute (DefaultMaven.java:173)
    at org.apache.maven.DefaultMaven.execute (DefaultMaven.java:101)
    at org.apache.maven.cli.MavenCli.execute (MavenCli.java:906)
    at org.apache.maven.cli.MavenCli.doMain (MavenCli.java:283)
    at org.apache.maven.cli.MavenCli.main (MavenCli.java:206)
    at jdk.internal.reflect.DirectMethodHandleAccessor.invoke (DirectMethodHandleAccessor.java:103)
    at java.lang.reflect.Method.invoke (Method.java:580)
    at org.codehaus.plexus.classworlds.launcher.Launcher.launchEnhanced (Launcher.java:283)
    at org.codehaus.plexus.classworlds.launcher.Launcher.launch (Launcher.java:226)
    at org.codehaus.plexus.classworlds.launcher.Launcher.mainWithExitCode (Launcher.java:407)
    at org.codehaus.plexus.classworlds.launcher.Launcher.main (Launcher.java:348)
Caused by: java.lang.RuntimeException: Could not find model-openapi.yaml on the classpath
    at io.swagger.v3.parser.util.ClasspathHelper.loadFileFromClasspath (ClasspathHelper.java:33)
    at io.swagger.v3.parser.util.RefUtils.readExternalClasspathRef (RefUtils.java:147)
    at io.swagger.v3.parser.ResolverCache.loadRef (ResolverCache.java:162)
    at io.swagger.v3.parser.processors.ExternalRefProcessor.processRefToExternalSchema (ExternalRefProcessor.java:89)
    at io.swagger.v3.parser.processors.SchemaProcessor.processReferenceSchema (SchemaProcessor.java:238)
    at io.swagger.v3.parser.processors.SchemaProcessor.processSchema (SchemaProcessor.java:59)
    at io.swagger.v3.parser.processors.ResponseProcessor.processResponse (ResponseProcessor.java:56)
    at io.swagger.v3.parser.processors.OperationProcessor.processOperation (OperationProcessor.java:86)
    at io.swagger.v3.parser.processors.PathsProcessor.processPaths (PathsProcessor.java:88)
    at io.swagger.v3.parser.OpenAPIResolver.resolve (OpenAPIResolver.java:72)
    at io.swagger.v3.parser.OpenAPIResolver.resolve (OpenAPIResolver.java:59)
    at io.swagger.v3.parser.OpenAPIV3Parser.resolve (OpenAPIV3Parser.java:240)
    at io.swagger.v3.parser.OpenAPIV3Parser.readContents (OpenAPIV3Parser.java:183)
    at io.swagger.v3.parser.OpenAPIV3Parser.readLocation (OpenAPIV3Parser.java:97)
    at io.swagger.parser.OpenAPIParser.readLocation (OpenAPIParser.java:16)
    at org.openapitools.codegen.plugin.CodeGenMojo.calculateInputSpecHash (CodeGenMojo.java:1012)
    at org.openapitools.codegen.plugin.CodeGenMojo.execute (CodeGenMojo.java:985)
    at org.apache.maven.plugin.DefaultBuildPluginManager.executeMojo (DefaultBuildPluginManager.java:126)
    at org.apache.maven.lifecycle.internal.MojoExecutor.doExecute2 (MojoExecutor.java:328)
    at org.apache.maven.lifecycle.internal.MojoExecutor.doExecute (MojoExecutor.java:316)
    at org.apache.maven.lifecycle.internal.MojoExecutor.execute (MojoExecutor.java:212)
    at org.apache.maven.lifecycle.internal.MojoExecutor.execute (MojoExecutor.java:174)
    at org.apache.maven.lifecycle.internal.MojoExecutor.access$000 (MojoExecutor.java:75)
    at org.apache.maven.lifecycle.internal.MojoExecutor$1.run (MojoExecutor.java:162)
    at org.apache.maven.plugin.DefaultMojosExecutionStrategy.execute (DefaultMojosExecutionStrategy.java:39)
    at org.apache.maven.lifecycle.internal.MojoExecutor.execute (MojoExecutor.java:159)
    at org.apache.maven.lifecycle.internal.LifecycleModuleBuilder.buildProject (LifecycleModuleBuilder.java:105)
    at org.apache.maven.lifecycle.internal.LifecycleModuleBuilder.buildProject (LifecycleModuleBuilder.java:73)
    at org.apache.maven.lifecycle.internal.builder.singlethreaded.SingleThreadedBuilder.build (SingleThreadedBuilder.java:53)
    at org.apache.maven.lifecycle.internal.LifecycleStarter.execute (LifecycleStarter.java:118)
    at org.apache.maven.DefaultMaven.doExecute (DefaultMaven.java:261)
    at org.apache.maven.DefaultMaven.doExecute (DefaultMaven.java:173)
    at org.apache.maven.DefaultMaven.execute (DefaultMaven.java:101)
    at org.apache.maven.cli.MavenCli.execute (MavenCli.java:906)
    at org.apache.maven.cli.MavenCli.doMain (MavenCli.java:283)
    at org.apache.maven.cli.MavenCli.main (MavenCli.java:206)
    at jdk.internal.reflect.DirectMethodHandleAccessor.invoke (DirectMethodHandleAccessor.java:103)
    at java.lang.reflect.Method.invoke (Method.java:580)
    at org.codehaus.plexus.classworlds.launcher.Launcher.launchEnhanced (Launcher.java:283)
    at org.codehaus.plexus.classworlds.launcher.Launcher.launch (Launcher.java:226)
    at org.codehaus.plexus.classworlds.launcher.Launcher.mainWithExitCode (Launcher.java:407)
    at org.codehaus.plexus.classworlds.launcher.Launcher.main (Launcher.java:348)
```
As you can see the problem is with resolved path. The correct path should be `./openapi/model-openapi.yaml` and not just `./model-openapi.yaml`.

Therefore you cannot use references to external files if these files are not in the root path of a jar.

##### openapi-generator version
openapi-generator 7.7.0 
it works fine with 7.6.0


##### OpenAPI declaration file content or url



##### Steps to reproduce
Try to build with `openapi-generator-maven-plugin`

##### Related issues/PRs
This PR https://github.com/OpenAPITools/openapi-generator/pull/18576 broke it.

This is connected to `java.net.URI.resolve()` that is used internally in `swagger-parser` https://bugs.openjdk.org/browse/JDK-8020755



##### Suggest a fix
Process only `inputSpec` if it starts with `jar:` or introduce a new configuration for disabling this processing.

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[BUG][MAVEN] Classpath $ref to external file are not resolved correctly #19073

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Uh oh!

[BUG][MAVEN] Classpath $ref to external file are not resolved correctly #19073

Description

Bug Report Checklist

Description

openapi-generator version

OpenAPI declaration file content or url

Steps to reproduce

Related issues/PRs

Suggest a fix

Metadata

Metadata

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Labels

Type

Fields

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