We allow any valid image reference format for the oci-layout, not just
limiting to name@digest, we additionally allow images of the form
name:tag@digest now.

The name of the reference is used to find the local directory to lookup
the store in, while the tag and digest are attached to a random identity
to generate the dummy reference sent to the oci-layout context.

This separation of the target to replace and the value to replace it
with ensures that any tag or digest set in the client is properly sent
across to the server. The tag is used when a digest was not specified,
and it is resolved in the context of the local directory before being
sent, using the same helpers as we use for the local cache expoter.

Signed-off-by: Justin Chadwell <me@jedevc.com>

Signed-off-by: Justin Chadwell <me@jedevc.com>