Call ListRepoNames from search code

Created by: asdine

This PR is the same as https://github.com/sourcegraph/sourcegraph/pull/16590 which was reverted because the externalRepository.ServiceType GraphQL API was empty. This is fixed by https://github.com/sourcegraph/sourcegraph/pull/16713 which ensures we always access repository information through a hydrated repository.

This PR changes the search code to use the new dedicated ListRepoNames method that only lists repository ids and names. Fixes #16219 (closed)