Add support for "exports" root subpath array shorthand

Summary:
Adds support for the `"exports"` root value being an array of strings, an edge-case legacy usage that technically remains in the Node.js 19.x spec — see [`"exports"` field (Type)](https://nodejs.org/docs/latest-v19.x/api/packages.html#exports).

Behaviour:

```js
  "exports": [
    "./lib/index.js"
  ],
```

expands to

```js
  "exports": {
    "./lib/index.js": "./lib/index.js"
  },
```

Changelog: **[Experimental]** Add compatibility with legacy Node.js "exports" array formats

Reviewed By: jacdebug

Differential Revision: D44338043

fbshipit-source-id: e9e719358c2610f4ae6d120d8312fcc647727da4

packages/metro-resolver/src/PackageExportsResolve.js

@@ -9,7 +9,12 @@
  * @oncall react_native
  */
 
-import type {ExportMap, FileResolution, ResolutionContext} from './types';
+import type {
+  ExportMap,
+  ExportsField,
+  FileResolution,
+  ResolutionContext,
+} from './types';
 
 import path from 'path';
 import InvalidModuleSpecifierError from './errors/InvalidModuleSpecifierError';
@@ -46,7 +51,7 @@ export function resolvePackageTargetFromExports(
    * to a package-relative subpath for comparison.
    */
   modulePath: string,
-  exportsField: ExportMap | string,
+  exportsField: ExportsField,
   platform: string | null,
 ): FileResolution {
   const raiseConfigError = (reason: string) => {
@@ -142,13 +147,23 @@ function getExportsSubpath(packagePath: string, modulePath: string): string {
  * See https://nodejs.org/docs/latest-v19.x/api/packages.html#exports-sugar.
  */
 function normalizeExportsField(
-  exportsField: ExportMap | string,
+  exportsField: ExportsField,
   raiseConfigError: (reason: string) => void,
 ): ExportMap {
   if (typeof exportsField === 'string') {
     return {'.': exportsField};
   }
 
+  if (Array.isArray(exportsField)) {
+    return exportsField.reduce(
+      (result, subpath) => ({
+        ...result,
+        [subpath]: subpath,
+      }),
+      {},
+    );
+  }
+
   const firstLevelKeys = Object.keys(exportsField);
   const subpathKeys = firstLevelKeys.filter(subpathOrCondition =>
     subpathOrCondition.startsWith('.'),

packages/metro-resolver/src/__tests__/package-exports-test.js

@@ -333,6 +333,29 @@ describe('with package exports resolution enabled', () => {
       });
     });
 
+    test('should expand array of strings as subpath mapping (root shorthand)', () => {
+      const logWarning = jest.fn();
+      const context = {
+        ...baseContext,
+        ...createPackageAccessors({
+          '/root/node_modules/test-pkg/package.json': JSON.stringify({
+            exports: ['./index.js', './foo.js'],
+          }),
+        }),
+        unstable_logWarning: logWarning,
+      };
+
+      expect(Resolver.resolve(context, 'test-pkg/index.js', null)).toEqual({
+        type: 'sourceFile',
+        filePath: '/root/node_modules/test-pkg/index.js',
+      });
+      expect(Resolver.resolve(context, 'test-pkg/foo.js', null)).toEqual({
+        type: 'sourceFile',
+        filePath: '/root/node_modules/test-pkg/foo.js',
+      });
+      expect(logWarning).not.toHaveBeenCalled();
+    });
+
     describe('should resolve "exports" target directly', () => {
       test('without expanding `sourceExts`', () => {
         expect(Resolver.resolve(baseContext, 'test-pkg/foo.js', null)).toEqual({

packages/metro-resolver/src/types.js

@@ -54,10 +54,12 @@ export type ExportMap = $ReadOnly<{
   [subpathOrCondition: string]: ExportMap | string | null,
 }>;
 
+export type ExportsField = string | $ReadOnlyArray<string> | ExportMap;
+
 export type PackageJson = $ReadOnly<{
   name?: string,
   main?: string,
-  exports?: string | ExportMap,
+  exports?: ExportsField,
   ...
 }>;