This commit removes the method ArtifactCache.get_artifact_fullname()
and replaces it with Element.get_artifact_name()

Given a key, we are now able to construct the full name of any of an
element's artifacts.

This object should be used when we want to handle artifact refs directly
from the command line.
An ArtifactElementError has also been added to _exceptions.py

* There is no need for this method to use a cas object.
* Search for artifact globs in the project's element path
* An artifact key is always 64 chars long

A CasBasedDirectory object of an artifacts logs can be obtained
with ArtifactCache.get_artifacts_log(). This ultimately calls
CASCache.get_top_level_dir() to obtain a CasBasedDirectory
of an artifact's subdirectory (or subdirectories).

The loading of elements and the handling of artifacts does not belong
in this module. Such logic should be invoked using the Stream API

This method has been moved to Stream, where is it used there exclusively.

This commit ensures that CASCache.list_refs(), and
ArtifactCache.list_artifacts(), can both handle glob expressions.

_classify_artifacts() no longer filters glob expressions by first
obtaining a list of ALL refs locally cached. We now only obtain refs
specified by the glob expression.

Furthermore, the Project.element_path is used to start searching for
globbed elements as opposed to the Project.directory.

Refactor artifact log command

See merge request !1101