search: refactor to not mutate resolveRepositories struct

Created by: rvantonder

Addresses https://github.com/sourcegraph/sourcegraph/pull/13164#discussion_r473796356

This sort of refactor is a little scarier, esp because resolveRepositories is big and finding the right place to create local vars... yeah...

Added this function to https://github.com/sourcegraph/sourcegraph/issues/13311 as a candidate to break up.