Produce mostly empty Microsoft.NETCore.App (dotnet/core-setup#5893)

 Produce mostly empty Microsoft.NETCore.App (dotnet/core-setup#5893)

* Produce mostly empty Microsoft.NETCore.App

Package is now empty with a platforms package dependency. Create Microsoft.NETCore.App.Insertion transport package for compatibility workarounds.

This commit also moves netcoreapp sfx generation to the new infrastructure to more easily handle the migration away from Microsoft.NETCore.App.

* Test workarounds for Msft.NETCore.App.Insertion

If tests use MNA, use MNAI.

* M.NETCore.App.Insertion => M.NETCore.App.Internal

It isn't just for insertion into core-sdk, we use it in core-setup too for now.

* Update dir.props: quote '/' and '\' in comment

Co-Authored-By: dagood <dagood@users.noreply.github.com>
* Comment on explicit .lib exclusion

Commit migrated from https://github.com/dotnet/core-setup/commit/0752a1798f8a089aa78796068a43b52ee1564aa5