ProjectImported OM should only exist in logger thread context

[JanKrivanek]

Context

#10959 (comment)

The _deferredProjectEvalIdToImportedProjects should be ensured to be called only from the BuildEventArgs handlers - so in a synchronized context. Then we can gt rid of any synchronization on it

What needs to be done

The handler of BuildCheckBuildEventHandler.HandleProjectEvaluationStartedEvent and ProcessProjectEvaluationStarted needs to be separated, as currently we cannot distinguish the situation "ProjectEvaluationStartedEvent occured, but it was received through logging infrastructure".

The reason for the 2 possible sources of the event (through the logging or directly from engine) is due to the fact that the BuildCheckManager can live in the worker node as well.

The ProjectImported data are to be received only via the logging infrastructure - so it should be handled only in the handler of BuildCheckBuildEventHandler.HandleProjectEvaluationStartedEvent. Since the logging infra guarantees synchronized (single threaded) delivery of events - we can then be sure that all handling of the ProjectImported data happens without concurrency risks and any synchronization (or usage of ConcurrentDictionary) can be removed. This should significantly simplify the code