MEF based approach had race issue where if aggregator created after source, aggregator will miss events fired before aggregator created.

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* This removes all references to NuGet.targets (which is obsolete and no longer exists)

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.

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

This reverts commit b37052d0.

The primary use of DefineConstants should be to append constant values,
not replace them.  Any time a replacement operation is used it prevents
us from establishing defines in our targets and pushing them down to all
affected projects.

We have some project references between projects that are either completely unnecessary or almost so, which have the nice side effect of creating build warnings due to version mismatches. Remove most of them, and move a single unit test helper next to the unit tests that are using it. (changeset 1408938)