Replace hard default of 'json' in sourceExts with package.json Watche…

…rImpl glob

Summary:
This follows up D9942449 (f9539ef) by replacing this internal and undocumented config merge with a more specific `**/package.json` glob pattern for Watchman within `metro-file-map`.

**Who this affects**: Only users who have overridden the `resolver.sourceExts` option and have removed `'json'` from this list.

**What this fixes**:
- **Changed**: Configuring `resolver.sourceExts` with a list that excludes `'json'` will now cause implicit `require`/`import`s of a `.json` file to fail (e.g. the resolver will **not** translate `import someData from './someData';` to `./someData.json` in an app's source code).
- **No change**: `package.json` files will always be internally recognised to be used by `metro-resolver`.

Changelog:

**[Breaking]** `.json` files will no longer be implicitly resolved if removed from `resolver.sourceExts`

Reviewed By: robhogan

Differential Revision: D37998169

fbshipit-source-id: e1d51c8fb28cd55577088e03ac689a030c7711bb

packages/metro-config/src/loadConfig.js

@@ -300,14 +300,6 @@ async function loadConfig(
 
   const overriddenConfig: {[string]: mixed} = {};
 
-  // The resolver breaks if "json" is missing from `resolver.sourceExts`
-  const sourceExts = configWithArgs.resolver.sourceExts;
-  if (!configWithArgs.resolver.sourceExts.includes('json')) {
-    overriddenConfig.resolver = {
-      sourceExts: [...sourceExts, 'json'],
-    };
-  }
-
   overriddenConfig.watchFolders = [
     configWithArgs.projectRoot,
     ...configWithArgs.watchFolders,

packages/metro-file-map/src/index.js

@@ -135,7 +135,7 @@ export type {
 // This should be bumped whenever a code change to `metro-file-map` itself
 // would cause a change to the cache data structure and/or content (for a given
 // filesystem state and build parameters).
-const CACHE_BREAKER = '1';
+const CACHE_BREAKER = '2';
 
 const CHANGE_INTERVAL = 30;
 const MAX_WAIT_TIME = 240000;
@@ -867,7 +867,12 @@ export default class HasteMap extends EventEmitter {
     const createWatcher = (root: Path): Promise<Watcher> => {
       const watcher = new WatcherImpl(root, {
         dot: true,
-        glob: extensions.map(extension => '**/*.' + extension),
+        glob: [
+          // Ensure we always include package.json files, which are crucial for
+          /// module resolution.
+          '**/package.json',
+          ...extensions.map(extension => '**/*.' + extension),
+        ],
         ignored: ignorePattern,
         watchmanDeferStates: this._options.watchmanDeferStates,
       });