qa workflow: Allow binary_artifacts to accept debian:binary-package artifacts

For regression tracking (#791), the blueprint says of the qa workflow:

The workflow is modified to also accept multiple debian:binary-package as input in binary_artifacts. This will require ensuring that the sub-workflows are able to accept a similar input.

I looked through the documentation of this workflow and all its child workflows and tasks. Some of them are already fine, and some accept debian:binary-package without documenting that they do so; but I think the following pieces need to be updated:

• Autopkgtest task (documentation only; perhaps also call ensure_artifact_categories)
• reverse_dependencies_autopkgtest workflow (documentation only)
• Piuparts task
• qa workflow (documentation only)