Ensure libs.yaml entries for all packages

Every library defined in the monorepo should have complete metadata.

Currently we rely on updating the global `libs.yaml` file.

In future maybe we should pull metadata from files in the library directory.

However, there are benefits to having the global file, such as triggering global review on updating core metadata.

For now, we should add a CI check to ensure that all packages in the repository have a `libs.yaml` entry. This will be particularly helpful when adding new libraries. If we add this check, we could drop the corresponding checklist item in the PR templates.