Updates to TFM support for reference pkg src generator

[mthalman]

In order to support packages with net6.0 TFM, we need to ensure that DisableImplicitFrameworkReferences is set to false (it's set to true by default) so that it compile against the existing targeting pack. There already existed code to set this to false but it was done globally for the entire project based on the presence of certain TFMs. But applying it for all TFMs is problematic because it can lead to build errors. For example, if applied to a netstandard2.0 TFM, you can get this error:

 error NETSDK1023: A PackageReference for 'NETStandard.Library' was included in your project. This package is implicitly referenced by the .NET SDK and you do not typically need to reference it from your project. For more information, see https://aka.ms/sdkimplicitrefs

The solution is to conditionally apply the setting of the DisableImplicitFrameworkReferences property based on the target framework. I've made changes to incorporate this with an existing PropertyGroup that sets the output path.

One additional change was made to add netcoreapp3.1 to the excluded TFMs.

Fixes dotnet/source-build#3251

[mthalman]

Sorry, @ViktorHofer. I shouldn't have enabled auto-merge to give everyone a chance to review. Let me know if further changes are needed.

[ViktorHofer]

Looks good 👍