Generalize how queries on _index
are handled at rewrite time
#52815
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Backport of #52486.
Since this change refactors rewrites, I also took it as an opportunity to adrress #49254: instead of returning the same queries you would get on a keyword field when a field is unmapped, queries get rewritten to a MatchNoDocsQueryBuilder.
This change exposed a couple bugs, like the fact that the percolator doesn't rewrite queries at query time, or that the significant_terms aggregation doesn't rewrite its inner filter, which I fixed.
Closes #49254