Search backend: move predicate expansion to predicate package

Created by: camdencheek

This moves (*searchResolver).expandPredicates() out of graphqlbackend and to internal/search/predicate:Expand(). Just continuing our slimming down of searchResolver.

We're getting close to being able to run search without any dependencies on graphqlbackend. I'm making a big push for this now because it seems achievable and it means that I can build repo-aware code monitors with extremely minimal changes (maybe none?) to our actual search code.

Test plan

Semantics preserving reorganization.