avoid marking non-existent project as indexed in the webapp

[vladak]

This change gets rid of the warning message for newly discovered projects during indexer run.

I contemplated various solutions:

• relaxing the indexed API call to ignore non-existent projects (lacks transparency / hides errors)
• propagating the list of discovered projects from the prepareIndexer() (too complex and would not reflect real webapp state)
• inferring project existence via project property call (clunky)

In the end the code retrieves the current list of projects known to the webapp. While this may seem a bit heavy, because this will be done for each indexed project, the webapp should withstand the traffic just fine and I wanted to avoid stashing the list in RuntimeEnvironment or somewhere in the Indexer and passing it around to IndexDatabase. Of course, I could have introduced new API end point to check for project existence, however deemed this unnecessary at this point.

Naturally, there is a time window between the check and the eventual mark-as-indexed operation, however let's assume the chance of the project to be deleted in that window is negligible.

While there, I refactored the code which performs the API call to mark the project as indexed into IndexerUtil.