Include NuGet.Packaging in ApiCompat.Task package

[ericstj]

Fixes issue raised #35467 (comment)

The SDK will filter which references will actually publish when PrivateAssets is set, but only for direct references. Here's that feature:

sdk/src/Tasks/Microsoft.NET.Build.Tasks/targets/Microsoft.NET.Sdk.Shared.targets

Lines 50 to 60 in 8dfacad

	<Target Name="_ComputePackageReferencePublish">
	<ItemGroup>
	<!-- PrivateAssets="All" means exclude from publish, unless Publish metadata is specified separately -->
	<PackageReference Publish="false"
	Condition="('%(PackageReference.PrivateAssets)' == 'All') And ('%(PackageReference.Publish)' == '')"/>
	<_ExcludeFromPublishPackageReference Include="@(PackageReference)"
	Condition="('%(PackageReference.Publish)' == 'false')" />
	</ItemGroup>
	</Target>

I'm not sure I really agree with that behavior -- since all the transitive references are still Private, but not filtered -- but it is what it is. This explains why the project previously set Publish metadata.

Here's the diff after this change, compared to the latest RC2 package: https://www.diffchecker.com/fmxLyhv6/

The removal of NuGet.Protocols and its closure is expected. Nothing was using that anymore after we removed the task that fetched the latest version from NuGet long ago. I intentionally dropped this in the original PR. There are no assembly references to this assembly in the task or its dependencies.