Other components will start to reply on cas modules, and not the artifact cache
modules so it should be organized to reflect this.

All relevant imports have been changed.

Part #802

Part of #802

List of methods moved
* Initialization check: made it a class method that is run in a subprocess, for
  when checking in the main buildstream process.
* fetch_blobs
* send_blobs
* verify_digest_on_remote
* push_method

Part of #802

Cas refactor

See merge request !1071