Add a dedicated method to fetch only a subset of repository columns

Created by: asdine

Currently, search code sometimes needs to fetch only a subset of repository information: the local id, name, the external id, and if the repo is private or not. https://github.com/sourcegraph/sourcegraph/blob/6b282f9176d54018402af43eb1d9e291364be2a2/internal/types/types.go#L88-L101

Since this need is very specific to the search code, add a dedicated method to the db.RepoStore type to only fetch these types, so that instead of returning a list of "standard" repos, it would return a dedicated struct containing only the information listed above.

This will prevent us from having to use the current structure in the whole codebase, and instead use the one defined in repo-updater, which is much simpler to use.