This reverts commit 3ae0f796, reversing
changes made to a2ec364d.

Closes #43728

Move DefinitionLocation declaration, add new parameters to GetConflictEditSpan and GetReferenceEditSpan

* Simplify analyzer assembly loaders, analyzer reference serialization

* Fold ReferenceSerializationService into SerializerService

* Add nullable annotations

* Compare analyzer ids instead of instances when calculating SkippedHostAnalyzersInfo

Currently, the `PeekableItemSource` assumes that if a language exports an `IGoToDefinitionService`, then it's non-null - however, this is incorrect, and can cause a null reference exception dialog in VS.

Here, we gracefully handle the potential null.

See #42814

* Eliminate WorkspaceTaskScheduler

* Move IAsynchronousOperationListener down to WorkspaceTaskSchedulerFactory

* Move IAsynchronousOperationListener to WorkspaceTaskQueue

* Split WorkspaceTaskSchedulerFactory

* Merge SimpleTaskQueue and WorkspaceTaskQueue into TaskQueue

* Add nullable annotations

* Make cancallation token explicit

Change all the direct uses of `Environment.FailFast` to instead use
`FailFast.Fail`. The latter has diagnostics which are useful for determining why
unit tests are failing.