Search backend: renames and reorganize

Created by: camdencheek

This is a followup from https://github.com/sourcegraph/sourcegraph/pull/35241 that takes the new changes and updates the code with some renames and reorganizations that take advantage of the new structure.

Stacked on #35241

Basic summary of the changes:

• We now have query.Q > query.Basic > query.Flat
• We have associated functions NewJob, NewBasicJob, and NewFlatJob
• Reorganizations try to make associated files hierarchical
• Comments are updated
• The method ToParseTree is renamed to ToQ to parallel ToBasic
• Updated tests to use NewJob so we have autogold tests that capture the full job tree rather than just a subtree.

Each commit is atomic, self-describing, and passes tests.

Test plan

I added a few tests and updated tests to capture the broader behavior. All the changes should be semantics-preserving and just narrow types, reorganize, or rename.