Make the CLI recognize CRNA projects

Summary:
1. Make running `react-native run-ios` or `react-native run-android`
inside a project created with Create React Native App run the
corresponding react-native-scripts command instead of failing.

    **Before**:
    <img width="762" alt="screen shot 2017-07-21 at 16 55 32" src="https://user-images.githubusercontent.com/497214/28467425-86b309c8-6e38-11e7-8946-139bda927d93.png"><img width="762" alt="screen shot 2017-07-21 at 16 55 52" src="https://user-images.githubusercontent.com/497214/28467436-8df02482-6e38-11e7-8a03-3fa664944cac.png">
    **After**:
    <img width="762" alt="screen shot 2017-07-21 at 16 52 15" src="https://user-images.githubusercontent.com/497214/28467522-e4bb6cae-6e38-11e7-97bb-9cfa9cb4dc67.png">

2. Make running `react-native link` inside a CRNA project display a
helpful error message.

    **Before**:
    <img width="762" alt="screen shot 2017-07-21 at 16 55 10" src="https://user-images.githubusercontent.com/497214/28467608-1d1781fa-6e39-11e7-9620-cc16c8b1b40f.png">
    **After**:
    <img width="762" alt="screen shot 2017-07-21 at 16 53 10" src="https://user-images.githubusercontent.com/497214/28467637-2cd6ed1a-6e39-11e7-8947-6df69b3f321e.png">

Fixes #14828.

* Run `react-native run-ios`, `react-native run-android` and `react-native link` in:
  * A CRNA project (screenshot above)
  * A traditional RN project (existing behaviour)
  * A folder that contains neither (existing behaviour)
Closes https://github.com/facebook/react-native/pull/15139

Differential Revision: D5498914

Pulled By: hramos

fbshipit-source-id: 94b6196e3451857bbaa45335a01643c89bed19a0

local-cli/util/findReactNativeScripts.js

@@ -0,0 +1,28 @@
+/**
+ * Copyright (c) 2015-present, Facebook, Inc.
+ * All rights reserved.
+ *
+ * This source code is licensed under the BSD-style license found in the
+ * LICENSE file in the root directory of this source tree. An additional grant
+ * of patent rights can be found in the PATENTS file in the same directory.
+ *
+ * @flow
+ */
+'use strict';
+
+const path = require('path');
+const fs = require('fs');
+
+function findReactNativeScripts(): ?string {
+  const executablePath = path.resolve(
+    'node_modules',
+    '.bin',
+    'react-native-scripts'
+  );
+  if (fs.existsSync(executablePath)) {
+    return executablePath;
+  }
+  return null;
+}
+
+module.exports = findReactNativeScripts;