[various] Update flutter/plugins links (#3256)
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).