Add netstandard2.1 placeholders

[ViktorHofer]

Fixes #212

• Remove netcoreapp placeholders when there's already a .NET Standard placeholder.
• Mark libraries as packable
• Simplify placeholder conditions
• Add a placeholder for dispatchproxy netcoreapp2.0 and netstandard2.1 and remove type forwards.

[ViktorHofer]

My sample application now passes with these changes when targeting netstandard2.1, netstanard2.0 and net462:

PS C:\temp\memory> dotnet build /bl
Restore complete (0.6s)
You are using a preview version of .NET. See: https://aka.ms/dotnet-support-policy
  memory netstandard2.1 succeeded (0.8s) → bin\Debug\netstandard2.1\memory.dll
  memory net462 succeeded (0.9s) → bin\Debug\net462\memory.dll
  memory netstandard2.0 succeeded (0.9s) → bin\Debug\netstandard2.0\memory.dll

The _HandlePackageFileConflict task doesn't report any conflicts anymore.

[carlossanlop]

Reviewed in a call with Viktor and confirmed the results are correct in conflict resolution.

• IsPackable enabled in the assemblies that need republishing.
• VersionPrefix not changed, that number was bumped in my last PR.
• AssemblyVersions, TargetFrameworks and their conditions modified as required, confirmed in the manual tests in the call.