feat: migrate to analyzer 9.0.0 API

- Replace Element2 API with Element API
- Update imports from element2.dart to element.dart
- Replace .firstFragment.name2 with .name
- Replace .firstFragment.metadata2.annotations with .metadata.annotations
- Replace .fields2 with .fields
- Replace .methods2 with .methods
- Remove duplicate imports in annotation_validator.dart and generated_class.dart
- Update test mocks to use Element instead of Element2

All generators now work correctly with analyzer 9.0.0:
- melos analyze: SUCCESS (0 issues)
- melos codegen: SUCCESS (13 outputs generated)
- melos bootstrap: SUCCESS (8 packages)

cherrypick_generator/lib/module_generator.dart

@@ -11,7 +11,7 @@
 // limitations under the License.
 //
 
-import 'package:analyzer/dart/element/element2.dart';
+import 'package:analyzer/dart/element/element.dart';
 import 'package:build/build.dart';
 import 'package:source_gen/source_gen.dart';
 import 'package:cherrypick_annotations/cherrypick_annotations.dart' as ann;
@@ -80,11 +80,11 @@ class ModuleGenerator extends GeneratorForAnnotation<ann.module> {
   /// See file-level docs for usage and generated output example.
   @override
   dynamic generateForAnnotatedElement(
-    Element2 element,
+    Element element,
     ConstantReader annotation,
     BuildStep buildStep,
   ) {
-    if (element is! ClassElement2) {
+    if (element is! ClassElement) {
       throw InvalidGenerationSourceError(
         '@module() can only be applied to classes.',
         element: element,