Move to MetadataAsSource as oppose of creating new project context

[vzarytovskii]

Chatted with @tmat about an issue we are having navigating to metadata, we need to move to using MetadataAsSource instead of creating an on-demand project context, as we do now.

Project contexts have different purpose. We should move off of it.

Additionally, we need to support Source Link go to definition in VS. @davidwengier may know more about the topic.

[tmat]

I think it would be best to define an internal interface between Roslyn and F# that allows F# to plug into our MAAS and Source Link GTD UI and implementation. Essentially just abstract what's specific to C#/VB in a way that F# can provide their implementation.