Fixes #78978

1. Move all full facade assemblies (which only contain type forwards)
   into src/libraries/shims.
2. Merge assemblies (ref+src) which typeforward to the same
   destination.
3. They inherently now don't produce a documentation file anymore (as
   shims only contain type forwards).

This minimizes the build graph (makes the libs build faster) and moves
the focus away from full facade assemblies that need to be kept for
compat reasons.

As discussed with Eric offline, typeforwarding to System.Runtime and
System.Xml.ReaderWriter makes it possible to collapse the following
ref and src projects into just src:

System.AppContext
System.Buffers
System.Diagnostics.Debug
System.Diagnostics.Tools
System.Globalization
System.Globalization.Calendars
System.IO.UnmanagedMemoryStream
System.Reflection
System.Resources.ResourceManager
System.Runtime.CompilerServices.Unsafe
System.Runtime.Extensions
System.Security.Principal
System.Text.Encoding
System.Threading.Timer
System.Xml.XmlDocument

The destination then handles the type fowarding to the private assembly
implementation.