Making babel to use the babelrc file if we find it

Summary:
This PR tells babel where the babelrc file is if it has been found in the project root. I know you guys are relying on babel to find it when you tell babel the filename and sourceFilename, this works fine in normal js/node projects, here is an example where this will not work.

We have a big repo that can contain a few react-native / js projects. Our build system is similar to BUCK and we keep our third party dependencies in a separate folder (having a global package.json at the project route is not feasible as everyone will have to install every apps dependencies even when they don't need them), the structure looks a bit like this:

`third_party/js/.` packages from node_modules
`common/js/` common code used by numerous apps, common components, services etc.
`my_app/native` the app

What's happening is something from `my_app/native` (where the .babelrc file is) is requiring something from `common/js` which is then requiring something from `third_party/js` and so babel has no idea what .babelrc file to use and
Closes #8131

Differential Revision: D4167844

Pulled By: hramos

fbshipit-source-id: 3569981e26ff8f8c632d8ae365a1f43a3483b13b

packager/transformer.js

@@ -37,11 +37,6 @@ const getBabelRC = (function() {
     // Let's look for the .babelrc in the first project root.
     // In the future let's look into adding a command line option to specify
     // this location.
-    //
-    // NOTE: we're not reading the project's .babelrc here. We leave it up to
-    // Babel to do that automatically and apply the transforms accordingly
-    // (which works because we pass in `filename` and `sourceFilename` to
-    // Babel when we transform).
     let projectBabelRCPath;
     if (projectRoots && projectRoots.length > 0) {
       projectBabelRCPath = path.resolve(projectRoots[0], '.babelrc');
@@ -58,6 +53,9 @@ const getBabelRC = (function() {
       // Require the babel-preset's listed in the default babel config
       babelRC.presets = babelRC.presets.map((preset) => require('babel-preset-' + preset));
       babelRC.plugins = resolvePlugins(babelRC.plugins);
+    } else {
+      // if we find a .babelrc file we tell babel to use it
+      babelRC.extends = projectBabelRCPath;
     }
 
     return babelRC;