feat(eslint): add support for ESLint v9

Update package.json to make eslint a peerDependency, so that the
user has control over which version to pick. Move the existing
eslint dependency to devDependencies so that tests work as expected.

Introduce a new test to ensure the plugin works with ESLint v9. To
do so without breaking v8 tests, install ESLint v9 under a different
alias, and use an ESM import mocking library to override the "eslint"
import.

The current directory needs to be changed during the ESLint v9 test
so that ESLint finds its config file properly. This requires ava
workerThreads to be disabled:

    TypeError [ERR_WORKER_UNSUPPORTED_OPERATION]: process.chdir() is not supported in workers

Author: emersion

packages/eslint/package.json

@@ -54,6 +54,7 @@
     "lint"
   ],
   "peerDependencies": {
+    "eslint": "^8.57.1||^9.0.0",
     "rollup": "^1.20.0||^2.0.0||^3.0.0||^4.0.0"
   },
   "peerDependenciesMeta": {
@@ -62,18 +63,21 @@
     }
   },
   "dependencies": {
-    "@rollup/pluginutils": "^5.0.1",
-    "eslint": "^8.57.1"
+    "@rollup/pluginutils": "^5.0.1"
   },
   "devDependencies": {
     "@rollup/plugin-node-resolve": "^15.0.0",
     "@rollup/plugin-typescript": "^9.0.1",
     "@types/eslint": "^8.56.12",
+    "eslint": "^8.57.1",
+    "eslint9": "npm:eslint@^9.37.0",
+    "esmock": "^2.7.3",
     "rollup": "^4.0.0-24",
     "typescript": "^4.8.3"
   },
   "types": "./types/index.d.ts",
   "ava": {
+    "workerThreads": false,
     "files": [
       "!**/fixtures/**",
       "!**/helpers/**",

packages/eslint/test/test.mjs

@@ -1,7 +1,11 @@
 import fs from 'fs';
-
 import { createRequire } from 'module';
+import process from 'process';
+import path from 'path';
+import { fileURLToPath } from 'url';
 
+import eslint9 from 'eslint9';
+import esmock from 'esmock';
 import test from 'ava';
 import nodeResolve from '@rollup/plugin-node-resolve';
 import { rollup } from 'rollup';
@@ -241,3 +245,36 @@ test('works with flat config', async (t) => {
 
   t.is(count, 1);
 });
+
+test.serial('works with ESLint v9', async (t) => {
+  // Load the plugin with an override to route 'eslint' imports to ESLint v9
+  const eslint = await esmock('current-package', {
+    eslint: eslint9
+  });
+
+  // ESLint v9 needs to be invoked with a flat config file in the current dir
+  const cwd = process.cwd();
+  const testDir = path.dirname(fileURLToPath(import.meta.url));
+  process.chdir(path.join(testDir, 'fixtures', 'flat-config'));
+
+  let count = 0;
+  try {
+    await rollup({
+      input: './undeclared.js',
+      plugins: [
+        eslint({
+          formatter: (results) => {
+            count += results[0].messages.length;
+            // eslint-disable-next-line prefer-destructuring
+            const { message } = results[0].messages[0];
+            t.is(message, "'x' is not defined.");
+          }
+        })
+      ]
+    });
+  } finally {
+    process.chdir(cwd);
+  }
+
+  t.is(count, 1);
+});