search: use repo pager job for text search jobs over repo sets
Created by: rvantonder
A very satisfying PR: this converts the textsearch jobs to be created and expressed as just independent Zoekt and Searcher jobs (after teasing out the different state and parameterizing over repos). The actual change is only about 30 lines, the rest is test values. Some fun observations in line.
Test plan
Intended to be semantics-preserving. Covered by unit tests and backend integration tests.