Add support for resolve references via packages.config.

Code review request from @terrajobst and @davkean. 
@weshaggard yeah, Mono defines a directory similar to `C:\Program Files (x86)\Reference Assemblies\Microsoft\Framework` with the usual `.NETFramework` and `.NETPortable` subfolders (and an env var to override where it looks for that dir).

Falling back to Profile7 should be fine for now.