search: put predicate substitution logic in own package

Created by: rvantonder

As in title.

We can't put this logic in internal/search/query because it depends on results (circular dependency). This makes sense since predicate substitution is a runtime behavior, and the predicate.go part of internal/search/query is all static. At some point I might find a way to structure this better, but right now it makes sense to have some predicate logic in its own package.

This is setup for changes to support dependencies predicate work.

Test plan

Semantics-preserving.