We were mixing the up-to-date status and in-progress status of an index task in `SemanticIndexManager`. This meant that a single `QueuedTask` in the task scheduler could be needed for eg. both preparation for editor functionality in a file of that target and to re-index a file in that target. This dual ownership made it unclear, which caller would be entitled to cancel the task. Furthermore, we needed to duplicate some logic from the preparation task dependencies in `SemanticIndexManager.prepare`.
To simplify things:
- Split the up-to-date status and the in-progress status into two different data structures
- Make the caller of `prepare` and `scheduleIndex` responsible for cancellation of the task it has scheduled. `TaskScheduler` might receive more scheduled tasks this way but the additional tasks should all be no-ops because the status is known to be up-to-date when they execute.
This makes it a lot easier to work on background indexing because you can easily see how background indexing is making progress.
Resolves#1257
rdar://127474057
