add metadata for multi-charm repos to 1.6 bundles

[ca-scribner]

This metadata is useful for driving CI, such as automatically triggering a release workflow for all charms in a bundle.

The metadata added here denotes, for any charm coming from a multi-charm repo, the path to the charm in the repository.

[ca-scribner]

This needs buyin from someone else on the nomenclature, otherwise it is ready