Our registry client is not currently in a good place to be used as the
reference OCI Distribution client implementation. But the registry proxy
currently depends on it. Make the registry client internal to the
distribution application to remove it from the API surface area (and any
implied compatibility promises) of distribution/v3@v3.0.0 without
breaking the proxy.
Signed-off-by: Cory Snider <csnider@mirantis.com>
