search: move job logic from 'run' -> 'job' package

Created by: rvantonder

Stacked on https://github.com/sourcegraph/sourcegraph/pull/31131.

Whoop whoop, now we can focus our energies inside this package, basically. Lots of stuff to simplify and a place for optimizer etc. to live.

Test plan

Semantics-preserving, covered by tests.