These are left over from an old way we used to build the compiler with
itself.

Fixes #5267.

* This updates all references to nuget.exe to use the copy in the root.
* This deletes all NuGet.config and NuGet.exe files that are not in the root.
* This removes all references to NuGet.targets (which is obsolete and no longer exists)

Cleaning up code. Only scripting uses the TryFindXmlDocumentationFile
helper. So moving it to that project and removing code that it doesn't
need.

These are added through project.json, we don't need these.

The portable tooling expects this package is added to portable projects,
but it wasn't in these.

- Undo some refactoring of CommonCompiler.ReadFileContent
- Remove LoadDirectiveResolver
- Make SourceReferenceResolver.ReadText detect encoding
- Make SourceReferenceResolver.ReadText Dispose its Stream
- Generate Diagnostic if source contains #load and SourceReferenceResolver
  is not specified in ParseOptions

The code was duplicated between scripting and workspaces because it is
partially non-portable.  Share the files by linking (and, unfortunately,
ifdef'ing) instead.