The netstandard.dll shim shouldn't be referenced inside the shared
framework as it's a compat shim that shouldn't be required to compose
the shared framework.

This removes the necessity of a separate RefPath build and improves
incremental build times as only the few OOB projects that require the
shim would need to rebuild.

Adding a netcoreapp2.0 configuration to SystemEvents as the source
assembly targets netcoreapp2.0 and this avoid the netstandard shim to
be used. The additional configuration shouldn't pose a problem as with
an eventual convergance of reference and src projects, we will likely
not mix universes between ref and src anymore and build the additional
matching universe assemblies.

There are a few OOB projects that require the netstandard.dll shim as
they intentionally reference projects that only expose a netstandard
configuration.

Adding netcoreapp3.1 assets to libraries to avoid the
netstandard.dll shim as a dependency. The secondary motive
for adding these configurations is to keep source and ref
configurations in sync to avoid weird cases where P2Ps
resolve assets from different assembly universes depending
on if it's a source or reference assembly.

The only libraries that still go through the netstandard.dll
shim are the Microsoft.Extensions ones.

Co-authored-by: Buyaa Namnan <[email protected]>