_artifact.py: Move element metadata caching to Artifact Class

As the artifact object now stores keys, it allows for the simplification
of metadata methods. Transitioning the metadata members to Artifact
also means the private accessor methods can be removed from Element
which reduces the overhead of syncing private variables across the
two classes via parameters.

With the key and metadata handling now being internalised within
Artifact, a temporary Artifact for the last_successful incremental
build is created for correctness. This should help guarantee that an
Artifact object is never used to access different artifacts.

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