By adding this method you now only have to pass a `*disco.Disco` object around in order to do discovery and use any configured credentials for the discovered hosts.

Of course you can also still pass around both a `*disco.Disco` and a `auth.CredentialsSource` object if there is a need or a reason for that!

The registry code has been moved into the new registry package. Remove
the duplicated code and use the new registry and registry/test packages.

Add GetModule for the cli to initialize from a regisry module source.

Storage.GetModule fetches a module using the same detection and
discovery as used by the normal module loading. The final copy is still
done by module.GetCopy to remove vcs files.