Product sources build against MyGet delivered framework contracts.

Previously, the product sources have been compiling against the
reference assemblies provided by the environment.  These were the
desktop reference assemblies.

This change adds package dependencies from each of our product
assemblies to .NET Core contract assemblies on a new MyGet feed at
https://www.myget.org/F/dotnet-corefx.  This change also modifies the
assembly projects to not implicitly reference the desktop reference
assemblies using the ImplicitlyExpandTargetFramework property.
Finally, this change explicitly references framework contract
assemblies in their location relative to the package root for
downloaded nuget dependencies.