Search backend: clean up repo resolver pt. 2

Created by: camdencheek

Builds off of https://github.com/sourcegraph/sourcegraph/pull/38598

1. Fix a typo Dependecies
2. Move dependency fetching logic into another method
3. Refactor goroutine group to use lib/group to avoid bad behavior when returning early from semaphore error
4. Make variable declarations more consise
5. Refactor to avoid mutating fields of a struct, preferring to build the fields then construct the struct with all the parts
6. Avoid constructing a gitserver client for every rev, instead preferring to store the client on the resolver, which also makes it possible to pass in a mock client.
7. Remove unused field OverLimit from Resolved

Test plan

Semantics-preserving, unit tests.