PKGProj referencing a project that only specifies TargetFramework doesn't work.

[ericstj]

I noticed this when looking at the Logging generator.

The generator only targeted <TargetFramework>netstandard2.0</TargetFramework>.

This caused

arcade/src/Microsoft.DotNet.Build.Tasks.TargetFramework.Sdk/src/build/Microsoft.DotNet.Build.Tasks.TargetFramework.Sdk.targets

Line 122 in e7ede87

<PackageTargetFrameworksList Include="$(TargetFrameworks)" />

to miss the single TargetFramework and silently drop the ProjectReference from the package.

Not sure if we should fix this with pkgproj going away, but it was pretty bad that it silently dropped. We should either error or handle the single $(TargetFramework).

cc @maryamariyan

[safern]

@ericstj more instances of this have happened in different ways. If an inbox project specifies TargetFramework it would not be binplaced into the shared framework, so I think we should error when TargetFramework is set but we should do that when we are not in a cross build? I don't know if that could cause people to not be able to build a single TFM using dotnet build, but we should definitely provide feedback to people if this is the case somehow.