To work around not getting some NuGet features that we won't get in time for 3.0, we need to provide a mostly empty Microsoft.NETCore.App package for the SDK to implement some fixes.

Proposal (thanks for this @dsplaisted!):

• Rename the existing Microsoft.NETCore.App package to something like Microsoft.NETCore.App.Internal or Microsoft.NETCore.App.Insertion
• Create a new Microsoft.NETCore.App package which has no assets, and only has a dependency on Microsoft.NETCore.Platforms

Issues this will help with:

• Package downgrades when version 1.x of Microsoft.NETCore.App is in the dependency graph: Restore fails if netstandard1.x package reference is present sdk#3044
• Package downgrades for Microsoft.NETCore.Platforms
• Platforms package showing up with warning icon in VS solution

Once Runtime Pack work is complete and those are used instead of Microsoft.NETCore.App, we can stop producing Microsoft.NETCore.App.Internal.

/cc @nguerrera