Remove public export for unstable-shared-subset.js (#23261)

The unstable-shared-subset.js file is not a public module — it's a
private module that the "react" package maps to when it's accessed from
the "react-server" package.

We originally added it because it was required to make our Rollup
configuration work, because at the time only "public" modules could act
as the entry point for a build artifact — that's why it's prefixed with
"unstable". We've since updated our Rollup config to support private
entry points, so we can remove the extra indirection.

packages/react/npm/react.shared-subset.js

@@ -0,0 +1,7 @@
+'use strict';
+
+if (process.env.NODE_ENV === 'production') {
+  module.exports = require('./cjs/react.shared-subset.production.min.js');
+} else {
+  module.exports = require('./cjs/react.shared-subset.development.js');
+}

packages/react/package.json

@@ -16,19 +16,18 @@
     "umd/",
     "jsx-runtime.js",
     "jsx-dev-runtime.js",
-    "unstable-shared-subset.js"
+    "react.shared-subset.js"
   ],
   "main": "index.js",
   "exports": {
     ".": {
-      "react-server": "./unstable-shared-subset.js",
+      "react-server": "./react.shared-subset.js",
       "default": "./index.js"
     },
     "./package.json": "./package.json",
     "./jsx-runtime": "./jsx-runtime.js",
     "./jsx-dev-runtime": "./jsx-dev-runtime.js",
-    "./unstable-shared-subset": "./unstable-shared-subset.js"
-
+    "./src/*": "./src/*"
   },
   "repository": {
     "type": "git",

packages/react/unstable-shared-subset.experimental.js → packages/react/src/ReactSharedSubset.experimental.js

@@ -37,4 +37,4 @@ export {
   useMutableSource as unstable_useMutableSource,
   useTransition,
   version,
-} from './src/React';
+} from './React';

scripts/rollup/bundles.js

@@ -88,7 +88,8 @@ const bundles = [
   {
     bundleTypes: [NODE_DEV, NODE_PROD],
     moduleType: ISOMORPHIC,
-    entry: 'react/unstable-shared-subset',
+    entry: 'react/src/ReactSharedSubset.js',
+    name: 'react.shared-subset',
     global: 'React',
     minifyWithProdErrorCodes: true,
     wrapWithModuleBoundaries: false,

scripts/rollup/forks.js

@@ -55,7 +55,7 @@ const forks = Object.freeze({
       // happens. Other bundles just require('object-assign') anyway.
       return null;
     }
-    if (entry === 'react' || entry === 'react/unstable-shared-subset') {
+    if (entry === 'react' || entry === 'react/src/ReactSharedSubset.js') {
       // Use the forked version that uses ES modules instead of CommonJS.
       return './packages/shared/forks/object-assign.inline-umd.js';
     }
@@ -82,7 +82,7 @@ const forks = Object.freeze({
     entry,
     dependencies
   ) => {
-    if (entry === 'react' || entry === 'react/unstable-shared-subset') {
+    if (entry === 'react' || entry === 'react/src/ReactSharedSubset.js') {
       return './packages/react/src/ReactSharedInternals.js';
     }
     if (!entry.startsWith('react/') && dependencies.indexOf('react') === -1) {