`Element.stage_artifact` and `Element.stage_dependency_artifacts` are no longer equivalent

Background

Since the addition of incremental workspaces Element.stage_dependency_artifacts has taken on more work than Element.stage_artifact. Since originally the former was supposed to only be nicer interface, this may well have an adverse effect on plugin API.

Task description

We should investigate if the change to Element.stage_dependency_artifacts has caused issues for plugins calling Element.stage_artifact.

Acceptance Criteria

We need to be absolutely sure that incremental workspaces can't break on a plugin using Element.stage_artifact.