Move empty-module.js from node-haste to metro-runtime

Summary: Moves `empty-module.js` into the `metro-runtime` package. This is used as a canonical empty module for some of Metro's resolver features.

Reviewed By: arushikesarwani94

Differential Revision: D30402880

fbshipit-source-id: 0c8b0d111ac8a313333d2d7d0d2241523a8827af

Author: motiz88

packages/metro/src/node-haste/DependencyGraph/assets/empty-module.js packages/metro-runtime/src/modules/empty-module.js

@@ -91,7 +91,9 @@ type Options<TModule, TPackage> = {|
 class ModuleResolver<TModule: Moduleish, TPackage: Packageish> {
   _options: Options<TModule, TPackage>;
 
-  static EMPTY_MODULE: string = require.resolve('./assets/empty-module.js');
+  static EMPTY_MODULE: string = require.resolve(
+    'metro-runtime/src/modules/empty-module.js',
+  );
 
   constructor(options: Options<TModule, TPackage>) {
     this._options = options;