search: introduce Zoekt job and repo partitioning

Created by: rvantonder

This creates a Zoekt backend job with a Run method. It inlines a bunch of repo resolution/index/unindex and zoektSearch logic and will let us remove a lot of other convoluted stuff. For simplicity I'm not removing existing things in this PR, and only putting the additive changes that gets us to the ideal state for this Zoekt job. I would add multiple commits to this PR but it's a lot of effort, so I'm doing it this way incrementally. This introduces an intermediate state of affairs, please see inline comments.

Test plan

Semantics-preserving, running backend tests.