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 
portable targeting pack 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 portable targeting pack
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.
After reading more of the comments from the PR hopefully the work @davkean is working on will eliminate these hint paths.