search: move private user repo look up to repos package
Created by: rvantonder
Stacked on https://github.com/sourcegraph/sourcegraph/pull/28449.
Mini epiphany: I think this function can actually live in the repos package, and this is useful now because (a) it de-bloats graphqlbackend
and (b) I can call it from unindexed
and symbols
packages, where the global search jobs will live and resolve private repos from (in a few upcoming PRs, we won't need to reference this functionality in graphqlbackend).
This PR just moves and exports the function.