Add support to disable downloading a runtime pack when RID is set

[safern]

Currently the SDK provides a hook to disable downloading the targeting pack: EnableTargetingPackDownload, however if a RuntimeIdentifier is set it attempts to download a runtime pack even if that property is set to false.

This has caused trouble in dotnet/runtime where we build a targeting/runtime pack (as a directory not nupkg) and then we override the PackageDirectory to point to the corresponding directory, so we never need to download either the targeting or runtime pack. However, now that we already moved our target framework to use net6.0 we had to add a KnownTargetFramework for net6.0 and we set the RuntimeFrameworkVersion to 6.0.0, since that runtime package doesn't exist in any of the feeds, we hit errors like:

/Users/santifdezm/repos/runtime/src/libraries/Microsoft.CSharp/tests/Microsoft.CSharp.Tests.csproj : error NU1102: Unable to find package Microsoft.NETCore.App.Runtime.browser-wasm with version (= 6.0.0)
/Users/santifdezm/repos/runtime/src/libraries/Microsoft.CSharp/tests/Microsoft.CSharp.Tests.csproj : error NU1102:   - Found 437 version(s) in dotnet5 [ Nearest version: 6.0.0-alpha.1.20420.3 ]
/Users/santifdezm/repos/runtime/src/libraries/Microsoft.CSharp/tests/Microsoft.CSharp.Tests.csproj : error NU1102:   - Found 396 version(s) in dotnet6 [ Nearest version: 6.0.0-alpha.1.20610.6 ]
/Users/santifdezm/repos/runtime/src/libraries/Microsoft.CSharp/tests/Microsoft.CSharp.Tests.csproj : error NU1102:   - Found 8 version(s) in nuget.org [ Nearest version: 5.0.1 ]
/Users/santifdezm/repos/runtime/src/libraries/Microsoft.CSharp/tests/Microsoft.CSharp.Tests.csproj : error NU1102:   - Found 0 version(s) in dotnet-eng
/Users/santifdezm/repos/runtime/src/libraries/Microsoft.CSharp/tests/Microsoft.CSharp.Tests.csproj : error NU1102:   - Found 0 version(s) in dotnet-tools
/Users/santifdezm/repos/runtime/src/libraries/Microsoft.CSharp/tests/Microsoft.CSharp.Tests.csproj : error NU1102:   - Found 0 version(s) in dotnet5-transport
/Users/santifdezm/repos/runtime/src/libraries/Microsoft.CSharp/tests/Microsoft.CSharp.Tests.csproj : error NU1102:   - Found 0 version(s) in dotnet6-transport

So we have had to add a workaround in a couple of places already to remove the package download with something like:

<Target Name="RemoveRuntimePackFromDownloadItem"
          Condition="'$(_UseLocalTargetingRuntimePack)' == 'true' and '$(RuntimeIdentifier)' != ''"
          AfterTargets="ProcessFrameworkReferences">
  <ItemGroup>
    <PackageDownload Remove="@(PackageDownload)"
                                       Condition="$([System.String]::Copy('%(Identity)').StartsWith('Microsoft.NETCore.App.Runtime'))" />
  </ItemGroup>
</Target>

It would be great if we could just set something like EnableRuntimePackDownload=false and this is skipped:

sdk/src/Tasks/Microsoft.NET.Build.Tasks/ProcessFrameworkReferences.cs

Lines 294 to 309 in 8ed60c6

	if (RuntimeIdentifiers != null)
	{
	foreach (var runtimeIdentifier in RuntimeIdentifiers)
	{
	if (processedPrimaryRuntimeIdentifier && runtimeIdentifier == this.RuntimeIdentifier)
	{
	// We've already processed this RID
	continue;
	}
	// Pass in null for the runtimePacks list, as for these runtime identifiers we only want to
	// download the runtime packs, but not use the assets from them
	ProcessRuntimeIdentifier(runtimeIdentifier, runtimePackForRuntimeIDProcessing, runtimePackVersion, additionalFrameworkReferencesForRuntimePack: null,
	unrecognizedRuntimeIdentifiers, unavailableRuntimePacks, runtimePacks: null, packagesToDownload, isTrimmable, includeInPackageDownload);
	}
	}

cc: @dsplaisted @ViktorHofer @ericstj @Anipik

[marcpopMSFT]

@safern I assume just disabling the download isn't enough as work would need to be done to pick up the runtime from the local build. We can put it in our backlog for 6.0 but don't know when we'll get to it but if ya'll want to contribute a PR, we'd be happy to take alook.

[ViktorHofer]

Right, in addition to disable the downloading of the runtime packs, we hook up the locally built runtime pack. I'm willing to send a PR, the change should be trivial.