The Flutter build process creates podspecs that are just shims pointing to the local Flutter framework, and which don't pass the linter since they aren't intended for publishing. When finding podspecs to lint, skip those.
This avoids incorrect failures when running on a tree that isn't clean (in particular, where macOS and/or iOS builds have happened), which is very common locally.
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).
