* 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* This removes all references to NuGet.targets (which is obsolete and no longer exists)

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 targets files were previously split into an open version in
Microsoft.CodeAnalysis.Toolset.Open and a closed version in build/. This
unifies the four targets into two new targets, VSL.Settings.targets and
VSL.Imports.targets, and moves all the targets from ..Toolset.Open into
the build/ folder.

Fell out of our normal cadence for this during the push for RTM. Now that we
had a bit more breathing room getting us back on track here.

This reverts 99317ad0.

This removes the majority of references to the deprecated Desktop
assemblies.  The only remaining ones are in the VSIX projects.  They
must remain until we actually delete the assemblies.

Upgrades System.Collections.Immutable to 1.1.34-rc and
System.Reflection.Metadata to 1.0.19-rc.