repo-updater: Convert Sources to send results on channel instead of returning a slice of repos

Created by: mrnugget

This is the preparation for solving https://github.com/sourcegraph/sourcegraph/issues/5145.

It converts the interface of all existing Sources to send results on a channel. It keeps the existing behavior in the separate Sources regarding error handling: some accumulate errors and some return early.

The next step is then to use the channel in the syncer.

For ease of reviewing I think it makes sense to merge this here first.