chore: switch eslint-config-eslint to the flat format (#17247)

* chore: switch eslint-config-eslint to the flat format

1. convert eslint-config-eslint to the flat format.
2. export eslint-config-eslint/eslintrc (used by testing).
3. upgrade `eslint-plugin-n` v16 (added flat config supports).
4. change eslint plugins peerDependencies => dependencies.
5. change lib/eslint/eslint.js, lib/eslint/flat-eslint.js test cases
to make the ci happy.

note: it's a breaking change for eslint-config-eslint.

refs: https://eslint.org/docs/latest/use/configure/configuration-files-new

* fix: add linterOptions

* chore: update comments as .eslintrc.js is no longer needed for testing

.eslintrc.js

@@ -6,8 +6,7 @@
  * Internally, ESLint is using the eslint.config.js file to lint itself.
  * This file is needed too, because:
  *
- * 1. There are tests that expect .eslintrc.js to be present to actually run.
- * 2. ESLint VS Code extension expects eslintrc config files to be
+ * 1. ESLint VS Code extension expects eslintrc config files to be
  *    present to work correctly.
  *
  * Once we no longer need to support both eslintrc and flat config, we will
@@ -63,7 +62,7 @@ module.exports = {
         "internal-rules"
     ],
     extends: [
-        "eslint"
+        "eslint/eslintrc"
     ],
     parserOptions: {
         ecmaVersion: 2021

eslint.config.js

@@ -29,20 +29,14 @@ const path = require("path");
 const internalPlugin = require("eslint-plugin-internal-rules");
 const eslintPluginRulesRecommendedConfig = require("eslint-plugin-eslint-plugin/configs/rules-recommended");
 const eslintPluginTestsRecommendedConfig = require("eslint-plugin-eslint-plugin/configs/tests-recommended");
-const { FlatCompat } = require("@eslint/eslintrc");
-const js = require("./packages/js");
 const globals = require("globals");
 const merge = require("lodash.merge");
+const baseConfig = require("eslint-config-eslint");
 
 //-----------------------------------------------------------------------------
 // Helpers
 //-----------------------------------------------------------------------------
 
-const compat = new FlatCompat({
-    baseDirectory: __dirname,
-    recommendedConfig: js.configs.recommended
-});
-
 const INTERNAL_FILES = {
     CLI_ENGINE_PATTERN: "lib/cli-engine/**/*",
     LINTER_PATTERN: "lib/linter/**/*",
@@ -81,7 +75,7 @@ function createInternalFilesPatterns(pattern = null) {
 }
 
 module.exports = [
-    ...compat.extends("eslint"),
+    ...baseConfig,
     {
         ignores: [
             "build/**",

package.json

@@ -116,7 +116,7 @@
     "eslint-plugin-eslint-plugin": "^5.1.0",
     "eslint-plugin-internal-rules": "file:tools/internal-rules",
     "eslint-plugin-jsdoc": "^38.1.6",
-    "eslint-plugin-n": "^15.2.4",
+    "eslint-plugin-n": "^16.0.0",
     "eslint-plugin-unicorn": "^42.0.0",
     "eslint-release": "^3.2.0",
     "eslump": "^3.0.0",

packages/eslint-config-eslint/README.md

@@ -22,12 +22,11 @@ npm install eslint-config-eslint --save-dev
 
 ## Usage
 
-In your `.eslintrc` file, add:
+In your `eslint.config.js` file, add:
 
-```json
-{
-    "extends": "eslint"
-}
+```js
+const eslintConfig = require("eslint-config-eslint");
+module.exports = eslintConfig;
 ```
 
 ### Where to ask for help?