Now that the telemetry API is consumable from OSS and the code is there, no
need to have a dedicated binary + vsix for it.

1. Reference a bunch of stuff from Nuget packages instead of VS.
2. Move to Update 2 versions of packages.
3. Now that we are using the packages, we're referencing
   Microsoft.VisualStudio.Shell.14.dll instead of .15 again.  This means
   that types that moved to Shell.Immutable.15 are not type forwarders.
   Luckily we didn't need anything from Shell.Immutable.15 in the project
   where it was a problem.

This brought along some references to Microsoft.VisualStudio.Threading
that we previously didn't have. Nothing was wrong with this, but it
meant I had to fix some namespace qualifications that were assuming the
"Threading" namespace was unambiguous.

To prevent projects across the tree from having different dependencies, we unify
external package dependencies by creating empty C# projects that represent them.
We then make use of project.json package inheritance (where a project inherits
all packages dependencies from their dependencies) to unify it across the tree.

No projects outside of this tree, should be directly referencing versions of
these dependencies manually.

For this check-in I've tackled Metadata, Reflection, and DiaSymReader. Will tackle
the rest in future commits.

This reverts commit b87b2712.

This fixes the ability to build with later versions of Visual Studio
on your machine, since some later versions are incorrectly embedding
type forwards to assemblies that they shouldn't be. The NuGet package
includes dependencies of

- Microsoft.VisualStudio.OLE.Interop
- Microsoft.VisualStudio.Shell.Interop
- Microsoft.VisualStudio.TextManager.Interop
- Microsoft.VisualStudio.Shell.Immutable.11.0

automatically, and so I remove those from the project files now too.

This reverts commit 760e1204.

This reverts commit e2fc5135.

Synchronize all corefx references to -23504 and bring in the
xunit coreclr test runner.

With Visual Studio 2015 Update 1 you should not specify -anycpu
in the runtimes section. Right now we'll have both to allow us to build
with and without Update 1, and we'll remove the old ones once we've
made the transition.

Update 1 also tries deploying implementations for some class libraries,
so I'm updating all our test and VSIX deployment class libraries to
now include implementations.

Updating Microsoft.DiaSymReader to v1.0.6, Microsoft.DiaSymReader.Native to v1.2.0, and System.Collections.Immutable to v1.1.37