Move full facade assemblies into src/libraries/shims

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.

Author: ViktorHofer

src/libraries/Microsoft.Win32.Registry/src/Microsoft.Win32.Registry.csproj

@@ -64,7 +64,6 @@
     <Compile Include="System\Security\AccessControl\RegistrySecurity.cs" />
   </ItemGroup>
   <ItemGroup>
-    <Reference Include="System.Buffers" />
     <Reference Include="System.Collections" />
     <Reference Include="System.Memory" />
     <Reference Include="System.Runtime" />