This commit also ensures that if we try to use the 'old' commands,
BuildStream will fail and instruct the user to use the new command.

Classifying artifact refs is useful for glob expansion
even in cases where an element target name would be inappropriate.

This allows for the creation of a CASRemote object using the
ArtifactCache API

With the addition of the artifact subcommands, which are able to
process artifact refs, we should not have to rely on CASCache
internals to handle artifacts. This commit ensures that the
pulling/pushing of elements (and their cache keys) is handled
by ArtifactCache.

This ensures that we can determine whether an artifact is locally cached
(as well as an element) using the ArtifactCache API.