Address review on MediaSourceCount batching

Rename GetItemsWithAlternateVersions to GetItemIdsWithAlternateVersions
across the interfaces and implementations since it returns ids. Return
the hashset straight from the query instead of materializing an array
first. Rename the DtoService guard to mayHaveAlternateVersions and
invert it so the computed path is the explicit case. Assert the media
source count value in the batch skip test and add a test covering an
item that is in the returned set still resolving to the correct count.

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