NuGet package licenses

[tmat]

This change must be merged after Preview 3, since it depends on changes to repos consuming Acade that were made to their master branches.

Updates the dotnet cli used for build to 2.1.503, which brings in new NuGet license features.
Removes the need for suppressing NU5125.

Since they are no longer passing PackageLicenseUrl this is a breaking change for projects that depend on Arcade targets when generating packages from .nuspec files.

NuGet supports PackageLicenseExpression or PackageLicenseFile (see docs). The former is essentially a well-known identifier for a license, such as MIT or Apache-2.0 for our repos. The later means including the license file in the package. The expression is preferable, since one doesn't need to check that the license file is exactly what it's supposed to be.

How do we make sure that the license file in the repo root matches the license specified by PackageLicenseExpression in Directory.Build.props file? This PR implements a simple content check for the licenses we need - it includes the expected license text files in Arcade SDK and then we compare the content and it must match modulo line breaks, whitespace. We require the license file in the root to be named license(.txt|.md|), ignoring case.

Each repository must specify either PackageLicenseExpression or PackageLicenseExpressionInternal. The former when an OSS license is used the later when closed source license is used.

The PR updates helpers for building packages from .nuspec. Instead of passing each msbuild property thru as a nuspec property and then listing all the metadata elements manually in the nuspec like so Arcade now defines a single property CommonMetadataElements that includes all the common nuspec metadata and can be used in the nuspec simply like so:

  <metadata>
    $CommonMetadataElements$
  </metadata>

Fixes #1955.
Fixes #1373.
Fixes #16.

[tmat]

@ericstj @chcosta @jmarolf @natemcmaster @dougbu @Eilon

[markwilkie]

@tmat - is it known which repos this will "break"?

[tmat]

@markwilkie All repos. They will need to specify PackageLicenseExpression.

[markwilkie]

ok - given the impact, I'll need to be sure and communicate this well before it's merged.

[tmat]

@markwilkie

given the impact, I'll need to be sure and communicate this well before it's merged.

Indeed. We need the DL for communicating breaking changes. This would be a good opportunity to set it up (I don't mind waiting a bit longer with merging this PR).

[tmat]

@markwilkie I figured out how to make this change in a non-breaking way.

[markwilkie]

Now that P3 is behind us, is this ready to go in @tmat ?

[tmat]

Yes. I'll merge.