_artifactcache modules: Adhere to policy on private symbols

Additionally:

  o This shares more code by creating ArtifactCache.get_artifact_fullname(),
    which is used for the extract directory relative path in both backends,
    and for the ostree "ref" in the ostree backend.

  o Further, this removes some redundant documentation in derived abstract
    methods, and clarifies in both backends which methods are abstract methods,
    we should only be documenting abstract method semantics in one place, where
    they are defined.

This is a part of issue #285