deprecate helper method in GetArtifactResourceImpl

It should be done using the PoddArtifactManager API instead of creating
a repository connection locally and then relying on PoddSesameManager
API
allow searching for artifact based on both artifact and version IRI in
PoddArtifactManager

Actual code wraps two of the PoddSesameManager calls for redundancy, as
we are fairly lax on people getting the version IRI right, and we
regularly fall back to the current version if they get it wrong.