fix: freezed_missing_mixin on private types due to leading underscore

Private types get their leading underscore stripped off, but the lint
wasn't accounting for that, so it would end up trying to change the type
name to one that does not actually exist.

Author: refi64

packages/freezed_lint/example/lib/mixin_name.dart

@@ -0,0 +1,8 @@
+import 'package:freezed_annotation/freezed_annotation.dart';
+
+part 'mixin_name.freezed.dart';
+
+@freezed
+class _FooModel with _$FooModel {
+  const factory _FooModel() = __FooModel;
+}

packages/freezed_lint/example/lib/mixin_name.freezed.dart

@@ -24,7 +24,9 @@ class MissingMixin extends DartLintRule {
       final annotation = freezedAnnotationChecker.hasAnnotationOfExact(element);
       if (!annotation) return;
 
-      final name = '_\$${element.name}';
+      final name = element.name.startsWith('_')
+          ? '_\$${element.name.substring(1)}'
+          : '_\$${element.name}';
       final withClause = node.withClause;
       if (withClause == null) {
         reporter.atElement(element, _code, arguments: [name]);