[various] Standardize the extension for Pigeon-generated Dart (#7029)

* Standardize on .g.dart

* Remove unused exclusion patterns

* Mark pigeons/ as dev-only in the tooling

* Version bumps

* Add missed files

* More new import fixes

script/tool/lib/src/common/package_state_utils.dart

@@ -171,6 +171,9 @@ Future<bool> _isDevChange(List<String> pathComponents,
       // The top-level "tool" directory is for non-client-facing utility
       // code, such as test scripts.
       pathComponents.first == 'tool' ||
+      // The top-level "pigeons" directory is the repo convention for storing
+      // pigeon input files.
+      pathComponents.first == 'pigeons' ||
       // Entry point for the 'custom-test' command, which is only for CI and
       // local testing.
       pathComponents.first == 'run_tests.sh' ||

script/tool/test/common/package_state_utils_test.dart

@@ -68,6 +68,8 @@ void main() {
         'packages/a_plugin/example/android/src/androidTest/foo/bar/FooTest.java',
         'packages/a_plugin/example/ios/RunnerTests/Foo.m',
         'packages/a_plugin/example/ios/RunnerUITests/info.plist',
+        // Pigeon input.
+        'packages/a_plugin/pigeons/messages.dart',
         // Test scripts.
         'packages/a_plugin/run_tests.sh',
         // Tools.