Search backend: move privateReposForActor to package where it's used
Created by: camdencheek
PrivateReposForActors is only used by zoekt code, and it's causing circular import issues blocking https://github.com/sourcegraph/sourcegraph/pull/38988. This just moves the function to where it's used.
Pulled out of #38988
Test plan
Semantics-preserving.