Account for new generated rules.json location in linter tool (#1956)

In preparation for the linter moving to the SDK, the `rules.json` file
is no longer generated to its own location. The site-www version is
better guaranteed to be up to date and match the `stable` release of
Dart.

experimental/linting_tool/lib/repository/api_provider.dart

@@ -8,15 +8,14 @@ import 'package:linting_tool/model/rule.dart';
 import 'package:yaml/yaml.dart';
 
 class APIProvider {
-  static const String _baseURL = 'https://dart-lang.github.io/linter';
-
   final http.Client httpClient;
 
   APIProvider(this.httpClient);
 
   Future<List<Rule>> getRulesList() async {
-    final response =
+    final response = await httpClient.get(Uri.parse(
-        await httpClient.get(Uri.parse('$_baseURL/lints/machine/rules.json'));
+      'https://raw.githubusercontent.com/dart-lang/site-www/main/src/_data/linter_rules.json',
+    ));
 
     if (response.statusCode == 200) {
       final data = json.decode(response.body) as List;

experimental/linting_tool/test/widget_test.dart

@@ -75,8 +75,8 @@ void main() {
         '''[{"name": "always_use_package_imports","description": "Avoid relative imports for files in `lib/`.","group": "errors","state": "stable","incompatible": [],"sets": [],"details": "*DO* avoid relative imports for files in `lib/`.\n\nWhen mixing relative and absolute imports it's possible to create confusion\nwhere the same member gets imported in two different ways. One way to avoid\nthat is to ensure you consistently use absolute imports for files withing the\n`lib/` directory.\n\nThis is the opposite of 'prefer_relative_imports'.\nMight be used with 'avoid_relative_lib_imports' to avoid relative imports of\nfiles within `lib/` directory outside of it. (for example `test/`)\n\n**GOOD:**\n\n```dart\nimport 'package:foo/bar.dart';\n\nimport 'package:foo/baz.dart';\n\nimport 'package:foo/src/baz.dart';\n...\n```\n\n**BAD:**\n\n```dart\nimport 'baz.dart';\n\nimport 'src/bag.dart'\n\nimport '../lib/baz.dart';\n\n...\n```\n\n"}]''';
 
     when(_mockClient.get(Uri.parse(
-            'https://dart-lang.github.io/linter/lints/machine/rules.json')))
+      'https://raw.githubusercontent.com/dart-lang/site-www/main/src/_data/linter_rules.json',
-        .thenAnswer(
+    ))).thenAnswer(
       (_) async => http.Response(responseBody, 400),
     );