Move query rewriting into search backend

Move query rewriting into search backend

Special:Search recently gained query rewriting behavior when the
original query returned no results.  We want to expose this query
rewriting behavior to both api and web requests.  Additionally we
want to be able to test different configurations of the query
suggestions.  This patch allows for both by moving the rewriting
from core into the search backend.

This defaults to enabled for Special:Search, and disabled for
ApiQuerySearch. Internal code that talks to the search backend
needs to specifically enable this feature.

Bug: T106888
Change-Id: I0a8f75759f9148f53358707369b8a7128215de86