Post co-hosting cleanup

[davidwengier]

This issue is a grab bag tracking things that were changed to enable us to work on cohosting, but we don't expect to be part of the final delivered solution.

• Remove DocumentVersionCache
  • Roslyn TextDocument is inherently versioned
• Remove DocumentState
  • Roslyn tracks document state in TextDocument, and the source generator does too via its pipeline.
• Remove ProjectState
  • Roslyn tracks project state in Project, and the source generator does the rest in its pipeline.
• Reduce the properties on DocumentSnapshot and ProjectSnapshot
  • If DocumentSnapshot remains in existence it should be a very light wrapper over a Roslyn document, and a convenient way to access data from the source generator. It should not store state, generally speaking.
• Maybe remove DocumentSnapshot/ProjectSnapshot
  • If the above removes enough properties, maybe DocumentContext is enough
• Alternative to the above, perhaps RemoteDocumentSnapshot is the type we use most often, and it allows access to the TextDocument for a Razor document, and the SourceGeneratedDocument for a generated document, and mapping between the two.
  • See Update document mapping to work with the source generator #11558 (comment)

[DustinCampbell]

Are any of the above needed for HTML?

[davidwengier]

Most of my day yesterday was spent staring at the wall trying to answer that question :)