Fix require('ReactFeatureFlags') in eslint-plugin-react-hooks www build (#36243)

PR #35951 added FB_WWW_DEV builds for eslint-plugin-react-hooks to get
www-specific feature flag values. However, the FB_WWW build uses the
full ReactFeatureFlags.www.js fork, which contains:

  const dynamicFeatureFlags = require('ReactFeatureFlags');

This is a www Haste module that only exists in the www runtime. Rollup
can't tree-shake CJS require() calls (they're assumed side-effectful),
so the bare require('ReactFeatureFlags') survives in the build output
even though the eslint plugin only uses the static eprh_* exports.

When the built artifact is synced to www at
scripts/lint/eslint/rules/eslint-plugin-react-hooks/index.js, Node.js
fails with "Cannot find module 'ReactFeatureFlags'" because Haste
modules aren't available in the Node.js lint environment.

Create a dedicated fork (ReactFeatureFlags.eslint-plugin.www.js) that
exports only the static eprh_* flags with www values, without the
require('ReactFeatureFlags') dependency. Wire it up in forks.js for the
eslint-plugin-react-hooks entry point.

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Co-authored-by: Eugene Choi <eugenechoi@meta.com>

Author: EugeneChoi4

packages/shared/forks/ReactFeatureFlags.eslint-plugin.www.js

@@ -0,0 +1,24 @@
+/**
+ * Copyright (c) Meta Platforms, Inc. and affiliates.
+ *
+ * This source code is licensed under the MIT license found in the
+ * LICENSE file in the root directory of this source tree.
+ *
+ * @flow strict
+ */
+
+// This fork provides www-specific feature flag values for
+// eslint-plugin-react-hooks without pulling in the full
+// ReactFeatureFlags.www.js fork. The full fork imports dynamic flags
+// via require('ReactFeatureFlags'), which is a www Haste module that
+// doesn't exist in Node.js environments where the ESLint plugin runs.
+//
+// Only eprh_* flags are needed by the ESLint plugin.
+
+export const eprh_enableUseKeyedStateCompilerLint: boolean = true;
+export const eprh_enableVerboseNoSetStateInEffectCompilerLint: boolean = true;
+export const eprh_enableExhaustiveEffectDependenciesCompilerLint:
+  | 'off'
+  | 'all'
+  | 'extra-only'
+  | 'missing-only' = 'extra-only';

packages/shared/forks/ReactFeatureFlags.www.js

@@ -114,13 +114,16 @@ export const enableFragmentRefsInstanceHandles: boolean = true;
 
 export const enableOptimisticKey: boolean = false;
 
-export const eprh_enableUseKeyedStateCompilerLint: boolean = true;
-export const eprh_enableVerboseNoSetStateInEffectCompilerLint: boolean = true;
+// These flags are only used by eslint-plugin-react-hooks, which has its own
+// fork at ReactFeatureFlags.eslint-plugin.www.js with the www-specific values.
+// Edit that file to change the www values for these flags.
+export const eprh_enableUseKeyedStateCompilerLint: boolean = false;
+export const eprh_enableVerboseNoSetStateInEffectCompilerLint: boolean = false;
 export const eprh_enableExhaustiveEffectDependenciesCompilerLint:
   | 'off'
   | 'all'
   | 'extra-only'
-  | 'missing-only' = 'extra-only';
+  | 'missing-only' = 'off';
 
 // Flow magic to verify the exports of this file match the original version.
 ((((null: any): ExportsType): FeatureFlagsType): ExportsType);

scripts/rollup/forks.js

@@ -163,6 +163,14 @@ const forks = Object.freeze({
               `Unexpected entry (${entry}) and bundleType (${bundleType})`
             );
         }
+      case 'eslint-plugin-react-hooks/src/index.ts':
+        switch (bundleType) {
+          case FB_WWW_DEV:
+          case FB_WWW_PROD:
+          case FB_WWW_PROFILING:
+            return './packages/shared/forks/ReactFeatureFlags.eslint-plugin.www.js';
+        }
+        return null;
       case 'react-test-renderer':
         switch (bundleType) {
           case RN_FB_DEV: