Search backend: clean up repo resolver

Created by: camdencheek

I'm working to move repo:contains operations work within the repo resolver. First up though, some cleanup.

1. Don't store options on the repo resolver. We should pass those in every time we call Resolve or Paginate, otherwise it's not really clear which we are using. Additionally, it makes a resolver re-usable for different calls.
2. Create a constructor for Resolver and use it
3. Make resolver fields private so it can't be constructed directly and we must go through the constructor.

Test plan

Semantics-preserving. Depending on unit tests.