Microsoft.Build.Commontypes.xsd contains PackageTargetFallback but not AssetTargetFallback

[ericstj]

/cc @mairaw @emgarten @dsplaisted
Related dotnet/docs#2816

I believe we recommend using AssetTargetFallback over PackageTargetFallback since it behaves better considering the entire package's asset groups rather than just evaluating it independently.

[emgarten]

//cc @mishra14

[dsplaisted]

I'd suggest removing PackageTargetFallback from the intellisense, since it can behave in ways you wouldn't expect.

I'm not sure at this point whether we should add AssetTargetFallback to the intellisense though. Hopefully by now it's not common to need to use it (we set it automatically to enable referencing .NET Framework projects from .NET Core / .NET Standard, but that doesn't mean people need to put it in their projects).

[rainersigwald]

It'll be required to support build-only (no reference) ProjectReferences, unfortunately, so I think it probably needs to go in.