search: factor out Zoekt job creation logic to functions

Created by: rvantonder

Now that the big ToSearchJob function block is "flat" over all kinds of state, we can finally start squashing shared concerns/state and keep separate ones... separate. This PR factors out functions for creating Zoekt jobs such that:

• shared state/compute values are shared by a jobBuilder type (see comments)
• different variants on the job are determined by using the appropriate constructor (global vs nonglobal) and parameterized by symbol vs text type.

This is just the start of factoring things out. The inconsistent naming and appearance of duplication is going to go away as the factoring becomes cleaner.

Context on why I'm doing this now: the state is finally approachable to cleanly separate, and I want to create those optimized Zoekt jobs. To do that, I need clear constructors to create Zoekt jobs in a place outside of this ToSearchJob function.

You can go by commit, but honestly, it's very mechanical and starts off with duplication, and then crunches logic into the constructors. Probably most time-efficient to just look at the full PR.

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Directions after this PR:

• Make Job types consistent (naming, reduce unneeded/unshared/duplicative state). I'm paying less attention to this right now because it's the slowest route to optimized Zoekt queries.
• Factor out constructors for other search jobs in ToSearchJobs as methods on jobBuilder
• Have ToSearchJobs be a method on jobBuilder (we won't construct jobBuilder in line).

Test plan

Semantics-preserving.