search: factor out zoekt query logic in query_converter.go
Created by: rvantonder
Stacked on https://github.com/sourcegraph/sourcegraph/pull/34549.
Moves the Zoekt query conversion into Zoekt package. This will probably make Zoekt friends @keegancsmith et al. happy, no need to go hunting down for this logic in internal/search
.
Now there are just a few dangling helper functions and types in query_converter.go
to clean up/factor.
Test plan
Semantics-preserving.