Symlink support for packager

Summary:
I saw we have quite a few user requests for symlink support...

**Test plan (required)**

1. Create a symlink in `node_modules` (for instance use `npm link`)
2. Run `npm start`
3. Profit!

**Code formatting**

Look around. Match the style of the rest of the codebase. See also the simple [style guide](https://github.com/facebook/react-native/blob/master/CONTRIBUTING.md#style-guide).

For more info, see the ["Pull Requests" section of our "Contributing" guidelines](https://github.com/facebook/react-native/blob/master/CONTRIBUTING.md#pull-requests).
Closes facebook/react-native#9009

Differential Revision: D3648828

Pulled By: matryoshcow

fbshipit-source-id: 99cf313bfa70324ca904fa6919ef112180974e9e

server/findSymlinksPaths.js

@@ -0,0 +1,15 @@
+const path = require('path');
+const fs = require('fs');
+
+module.exports = function findSymlinksPaths(lookupFolder) {
+  const timeStart = Date.now();
+  const folders = fs.readdirSync(lookupFolder);
+  const resolvedSymlinks = folders.map(folder => path.resolve(lookupFolder, folder))
+    .filter(folderPath => fs.lstatSync(folderPath).isSymbolicLink())
+    .map(symlink => path.resolve(process.cwd(), fs.readlinkSync(symlink)));
+  const timeEnd = Date.now();
+
+  console.log(`Scanning ${folders.length} folders for symlinks in ${lookupFolder} (${timeEnd - timeStart}ms)`);
+
+  return resolvedSymlinks;
+};

server/server.js

@@ -12,12 +12,16 @@ const chalk = require('chalk');
 const formatBanner = require('./formatBanner');
 const path = require('path');
 const runServer = require('./runServer');
+const findSymlinksPaths = require('./findSymlinksPaths');
 
 /**
  * Starts the React Native Packager Server.
  */
 function server(argv, config, args) {
-  args.projectRoots = args.projectRoots.concat(args.root);
+  args.projectRoots = args.projectRoots.concat(
+    args.root,
+    findSymlinkPaths(path.resolve(process.cwd(), 'node_modules'))
+  );
 
   console.log(formatBanner(
     'Running packager on port ' + args.port + '.\n\n' +