codeintel: Cull repository indexing list via search contexts

Created by: efritz

Fixes #26060 (closed). This is partial effort towards limiting our available corpus of auto-index repositories. We currently use repo groups and are trying to switch to search contexts for the same purpose.

This PR culls the set of repos that are candidates for auto-indexing by choosing only those that exist in any search context. In the future we'll want to interface more closely with this (attaching search contexts to policies, etc).

This should at least put us on our way to unblock search from deprecating repo groups (there's a bit more work to land but it's already prepared).