[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/pubspec_check_command.dart

@@ -227,9 +227,12 @@ class PubspecCheckCommand extends PackageLoopingCommand {
             .add('The "repository" link should end with the package path.');
       }
 
-      if (pubspec.repository!.path.contains('/master/')) {
+      if (!pubspec.repository!
+          .toString()
+          .startsWith('https://github.com/flutter/packages/tree/main')) {
         errorMessages
-            .add('The "repository" link should use "main", not "master".');
+            .add('The "repository" link should start with the repository\'s '
+                'main tree: "https://github.com/flutter/packages/tree/main".');
       }
     }