This builds support for tracking the effect of changes to imports on
other documents, and completes our model for being able to keep
generated code up to date.
- Prior to this we had a `BackgroundDocumentGenerator` that would constantly be updating generated code containers. With this changes we've changed the details in how a `GeneratedCodeContainer` can be mutated. It can now be touched from any thread and is updated when an underlying `DocumentSnapshot` has available content. However, if a generated output comes through that's older then the last seen output we no-op.
- Removed `BackgroundDocumentGenerator` SetOutput logic.
- Updated tests to react to new behavior of `GetGeneratedOutputAsync`.
- Added new test to verify getting generated output results in the setting of the host documents output.
Ryan Nowak
N. Taylor Mullen