search: group job creation by kind of search

Created by: rvantonder

This just reorganizes the blocks so that toplevel conditionals group jobs by the kind they create, so:

if (text search) {
 ... Zoekt Global, normal Zoekt, Searcher job creations
}

if (symbol search) {
 ... Zoekt Global, normal Zoekt, symbol search creations
}

Previously, whether a search was global or not were a mix of the toplevel conditionals like if repoUniverSearch { ... } for creating text/search jobs. I.e., it was grouped by backend/optimization, not the kind of search (text vs symbol).

The new organization is going to make it easier to remove the confusing relations between repoUniverseSearch, skipRepoSubsetSearch, and runZoektOverRepos which came from trying to disentangle modes.

Test plan

Semantics-preserving.