Extract common packagereferences for MSBuild tasks that ship in the SDK

[rainersigwald]

Several projects in this repo build MSBuild task assemblies. They all have clauses in their project file like

sdk/src/Tasks/Microsoft.NET.Build.Tasks/Microsoft.NET.Build.Tasks.csproj

Lines 83 to 85 in 39856f4

	<PackageReference Include="System.Collections.Immutable" VersionOverride="$(SystemCollectionsImmutableToolsetPackageVersion)" />
	<PackageReference Include="System.Reflection.Metadata" VersionOverride="$(SystemReflectionMetadataToolsetPackageVersion)" />
	<PackageReference Include="System.Reflection.MetadataLoadContext" VersionOverride="$(SystemReflectionMetadataLoadContextToolsetPackageVersion)" />

That serve to keep the .NET Framework task unified with the dependencies that MSBuild has. Unfortunately they can differ.

We should extract the concept of "this project builds SDK MSBuild tasks" to an imported SDKDeliveredMSBuildTask.props file that includes the references like that, and import it in the various projects that build tasks.

You can find those projects by searching for SystemReflectionMetadataToolsetPackageVersion and other properties to make sure you've found them all.