Migrating charm libraries management to the new library declaration system

[Guillaumebeuzeboc]

Migrate to this new declaration model: https://juju.is/docs/sdk/find-and-use-a-charm-library#heading--fetch-a-charm-library

          > Maybe since you are migrating the libraries it's worth it to migrate to the new library declaration system: https://juju.is/docs/sdk/find-and-use-a-charm-library#heading--fetch-a-charm-library

Let's do it in a follow-up pr once the other pending prs are merge 👍 .

Originally posted by @artivis in #24 (comment)