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.

• [x] 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

[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 :)

[davidwengier]

This is mostly done, and anything else will naturally fall out of deprectation.