[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).

script/tool/lib/src/format_command.dart

@@ -16,8 +16,8 @@ import 'common/process_runner.dart';
 
 /// In theory this should be 8191, but in practice that was still resulting in
 /// "The input line is too long" errors. This was chosen as a value that worked
-/// in practice in testing with flutter/plugins, but may need to be adjusted
-/// based on further experience.
+/// in practice via trial and error, but may need to be adjusted based on
+/// further experience.
 @visibleForTesting
 const int windowsCommandLineMax = 8000;