search: factor out job logic in query_converter.go

Created by: rvantonder

Want to get rid of query_converter.go. This file was a bit of a dumping/holding ground to concentrate query conversion logic + Zoekt query conversion. About half of this file belongs in job (which does query conversion to the TextPatternInfo type, hopefully soon to go away) and the other half belongs in Zoekt (which does query-to-zoekt conversion), similar to how we've structure commit/symbol query conversion in those backend packages.

I had to duplicate some helpers to move the tests or avoid circular dependencies, they will go away/unify after splitting things off.

Test plan

Semantics-preserving, just moving things around.