Change $documentSourceInternalMongotRemote to actually register as an internal document source

Currently we don't check that users are not running $_internalMongotRemote, and this might have consequences for sharded clusters after the sharded facets project. Would need to investigate more to see if there's actually a problem.

It seems like pre 'storedSource' this was a way to warm up Lucene Indexes. While this was not documented and not told to users, storedSource is currently a viable alternative.