Consider exposing Microsoft.Build internals to MSBuild.exe

Today, shared source causes many of the same types to be compiled into `Microsoft.Build.dll` and `MSBuild.exe`. That caused a lot of confusion here: https://github.com/microsoft/msbuild/pull/4916#discussion_r364990547.

We should figure out if there's a strong reason not to just let MSBuild see Microsoft.Build's internals, which would allow us to remove much of the double-compilation.