search: prefer MatchCount to ResultCount

Created by: rvantonder

In the GQL API we say:

# DEPRECATED: Renamed to 'matchCount' for less ambiguity.
resultCount: Int! @deprecated(reason: "renamed to matchCount for less ambiguity")

So, let's make ResultCount calls be like those using MatchCount.

Background: I want to report more accurate match counts for structural search (see #9426) and doing some cleanups along the way.