Allow metadata on FrameworkReference such that no references are actually raised

[wtgodbe]

I just opened dotnet/aspnetcore#65478 to solve a problem in aspnetcore: We build our projects with ProjectReferences, but for OOB packages that have references to SharedFx-only libraries, those packages need to wind up with a FrameworkReference to Microsoft.AspNetCore.App in their .nuspec (we used to achieve this by manually re-writing the .nuspec post-build).

The new approach is to add the FrameworkReference so that restore generates a project.assets.json w/ the FrameworkReference, then hook a target before ResolveFrameworkReferences that removes it, so it doesn't actually get used. We also have to set IsTransitiveFrameworkReference=true on the FrameworkRef so that pruning warnings don't fire. It would be better if there was some metadata we could set on the FrameworkReference (like "NuspecOnly" or "DoNotRaiseAssets") so that it'd wind up in the assets file/nuspec, but its references wouldn't actually get used.

I tried "ExcludeAssets=all" but the compiler still gets references from the FrameworkReference.

[wtgodbe]

CC @dsplaisted @ericstj

[ericstj]

I think ExcludeAssets is an interesting suggestion here and reflects the desired behavior. We want a framework reference, but we don't want the SDK to raise any assets from it. The only downside with calling it ExcludeAssets here is that might imply to users that the behavior is persisted to the package but it wouldn't be unless we asked NuGet for a similar feature. It could be called something else to avoid that implication. Or we could just ignore that implication since the users of this feature are for people building frameworks and packages -- which is just us.