search: pass repos to structural search job

Created by: rvantonder

This removes the RepoFetcher idea I used for the first search job (structural search). It turns out after separating resolved repos from args and exposing it to the structural search Run method works now. But the real motivation is that @tsenart and @stefanhengl have in mind to pass a cursor for fetching repos for the Run argument (basically a paginated "RepoFetcher" that's much cleaner), and this change will help with that effort when the type changes, and also structural search job will benefit.