Updates links and references to `flutter/plugins` to use `flutter/packages` instead, including making the `repository` pubspec.yaml check stricter in the repo tooling to ensure all packages are pointing to the right place.
Exceptions to the updates are:
- Changelog entries.
- Links to pull requests.
This will re-publish all the moved plugins, thus fixing the current redness of `release` (due to the current versions not being tagged in this repository).