search: reduce state dependencies of SearchFilesInRepos

Created by: rvantonder

This converts the signature of SearchFilesInRepos to only take the arguments needed to do a Zoekt query, Searcher query, and a "mode" setting, instead of all of TextParameters. It basically just lifts out the construction of smaller state to all callers of SearchFilesInRepos and SearchFilesInReposBatch. This action gets the construction of zoekt and searcher queries (i.e., jobs) closer to being upfront. It's a significant change, but not earth shattering (it's semantics-preserving). See inline comments for more.

Here are the call sites that were updated to take the new, reduced set of argument state:

SearchFilesInRepos - 2 call sites, one in aggregator, and one that is SearchFilesInReposBatch. I just propagated it up for SearchFilesInReposBatch as well:

SearchFilesInReposBatch - 4 call sites in test, 2 call sites in code (suggestions and repoHasFile logic).

All updated callsites, in other words: All call sites

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I don't know what's going on with our CI. This passed integration tests but there's something wonky with Go tests. Will come back to it later. Want this up for review.