FileTracker should be consumed from a package

[rainersigwald]

The source code for Tracker.exe and FileTracker.dll is not open source, but those assemblies are part of the MSBuild binary distribution. We should build them internally and make them available as a package so that the built-from-GitHub version of MSBuild can include them and ship them back into Visual Studio (or other consumers). Currently tests fall back to the installed versions, which makes testing coordinated changes very difficult.

[rainersigwald]

This doesn't seem that important at the moment.