Since charmcraft 2.5, all properties are to be written in
charmcraft.yaml. But when the charm is generated, the artifact will
still contain the metadata.yaml. Adding a module to read the artifact's
metadata.
Two benefits from this:
- don't have to check whether it's a charm with charmcraft.yaml or
metadata.yaml
- upload oci images written in the artifact's resources section
Change-Id: I971973859d0ef76840b5c1d60098a31e55a8d28b
652e8b3478