repo-updater: Take `ExternalRepo` into account in `RepoLookup`

Created by: mrnugget

In https://github.com/sourcegraph/sourcegraph/pull/3967#discussion_r283293070 @tsenart noticed that we do not take the ExternalRepo field of protocol.RepoLookupArgs into account:

https://github.com/sourcegraph/sourcegraph/blob/58f657de688b0905ca026fbb9c2d1a60086cd230/cmd/repo-updater/repoupdater/server.go#L342-L344

The protocol.RepoLookupArgs matches the external_* columns we have in the repo table:

https://github.com/sourcegraph/sourcegraph/blob/58f657de688b0905ca026fbb9c2d1a60086cd230/pkg/api/api.go#L52-L73

There are multiple places in the codebase that make use of this field:

• repos_vcs.go#L57:68
• repository.go#L122:66
• repository_mirror.go#L67:80

We can either:

1. Only use ExternalRepo.ServiceType in the Kinds field of repos.StoreListReposArgs
2. Extend the store with a GetRepo method that takes in a GetRepoArgs that has ExternalId, ExternalServiceType, ExternalServiceID fields