repo-updater: No private repos on sourcegraph.com

Created by: tsenart

This commit makes it so that repo-updater's repo lookup endpoint returns a "not found" for any private repo in either github.com or gitlab.com.

This is better than only relying on the access token in the external service not having access to private repos and failing to clone those repos in gitserver.

This change does not affect the normal syncer code paths, so we'll have to consider how to not support private code (for now) there, before we roll out everything from #12699.

Additional context: https://sourcegraph.slack.com/archives/C07KZF47K/p1597091562260500