Remove use_modular_headers! from Podfiles

[loic-sharma]

⚠️ This PR does not update packages' versions as it only affects unpublished parts of example apps and tests.

We added use_modular_headers! to our Podfiles as we originally planned to phase out use_frameworks! (see flutter/flutter#42204). However, our plans have now changed and we are instead phasing out CocoaPods entirely in favor of Swift Package Manager.

CocoaPods's use_frameworks! and use_modular_headers! are two different overlapping options that should not be used together. This change removes the use_modular_headers! from example apps' Podfiles. This change does not affect packages themselves.

Part of flutter/flutter#156259

Pre-launch Checklist

• I read the Contributor Guide and followed the process outlined there for submitting PRs.
• I read the Tree Hygiene page, which explains my responsibilities.
• I read and followed the relevant style guides and ran the auto-formatter. (Unlike the flutter/flutter repo, the flutter/packages repo does use dart format.)
• I signed the CLA.
• The title of the PR starts with the name of the package surrounded by square brackets, e.g. [shared_preferences]
• I linked to at least one issue that this PR fixes in the description above.
• I updated pubspec.yaml with an appropriate new version according to the pub versioning philosophy, or this PR is exempt from version changes.
• I updated CHANGELOG.md to add a description of the change, following repository CHANGELOG style, or this PR is exempt from CHANGELOG changes.
• I updated/added relevant documentation (doc comments with ///).
• I added new tests to check the change I am making, or this PR is test-exempt.
• All existing and new tests are passing.

If you need help, consider asking for advice on the #hackers-new channel on Discord.

[jmagman]

LGTM once the test passes

  Expected: equals [
              ProcessCall:"which pod" in /packages,
              ProcessCall:"pod lib lint /packages/plugin1/ios/plugin1.podspec --configuration=Debug --skip-tests --use-modular-headers --use-libraries" in /packages,
              ProcessCall:"pod lib lint /packages/plugin1/ios/plugin1.podspec --configuration=Debug --skip-tests --use-modular-headers" in /packages
            ] ordered
    Actual: [
              ProcessCall:"which pod" in /packages,
              ProcessCall:"pod lib lint /packages/plugin1/ios/plugin1.podspec --configuration=Debug --skip-tests --use-libraries" in /packages,
              ProcessCall:"pod lib lint /packages/plugin1/ios/plugin1.podspec --configuration=Debug --skip-tests" in /packages
            ]
     Which: at location [1] is ProcessCall:<"pod lib lint /packages/plugin1/ios/plugin1.podspec --configuration=Debug --skip-tests --use-libraries" in /packages> instead of ProcessCall:<"pod lib lint /packages/plugin1/ios/plugin1.podspec --configuration=Debug --skip-tests --use-modular-headers --use-libraries" in /packages>
  
  package:matcher                             expect
  test/podspec_check_command_test.dart 144:7  main.<fn>.<fn>

[stuartmorgan-g]

⚠️ This PR only changes unpublished parts of example apps and tests. I have added the override: no versioning needed label.

Could you link to the failing run that led to adding this label? CI should not have flagged this as needing a version change.

[loic-sharma]

@stuartmorgan Ah, I misunderstood the content at the bottom of this section to mean "Add the override label once you've verified your change meets these conditions": https://github.com/flutter/flutter/blob/master/docs/ecosystem/contributing/README.md#version

I'll update this README to clarify this label should only be added if the CI check fails: flutter/flutter#156342

[chunhtai]

LGTM

[LouiseHsu]

in_app_purchase, local_auth, and pointer_interceptor lgtm!

[loic-sharma]

This is now ready for review! The webview test failure that affected this change was also fixed by flutter/flutter#157449

[stuartmorgan-g]

LGTM