search: add indexedRepoRevs for Sourcegraph Zoekt mapping

Created by: keegancsmith

This PR introduces indexedRepoRevs which is responsible for creating and maintaining the maps for indexed search. We currently have an invariant that needs to be maintained between the zoekt and sourcegraph representations. This moves the responsibility solely into indexedRepoRevs, rather than spread out across functions.

Can be reviewed commit by commit. No commit adds/changes functionality, they are all refactorings to get to the final state of nicer code :)

Part of #6728