We should add the following package dependencies to the "Empty" Microsoft.NETCore.App package (see #3528):

• Microsoft.NETCore.Targets
  • This will help fix Restore fails if netstandard1.x package reference is present sdk#3044
  • This should be to the corresponding 3.0 version of this package
  • We should also remove the runtime.json file from the Microsoft.NETCore.Targets package. It contains "supports" information which is not used anymore.
• NETStandard.Library version 2.0.3
  • This will prune the 1.x contracts from the dependency graph if they are coming from a package that has a dependency on NETStandard.Library 1.6.1 (or 1.6.0).
  • .NET Standard 2.1 uses a targeting pack, so we don't have a 2.1 package to depend on
  • We could create an "empty" 2.1 NETStandard.Library package if there's a reason to do so, but at this point I'm not sure there is