add support for installing react-scripts using npm link

Created by: tabrezm

When using npm link to depend on a forked version of react-scripts, the paths are set up incorrectly such that the source files in the react-scripts template get used instead of the actual source files from the app.

Repro steps:

1. Create a package using the public version of create-react-app.
2. Clone create-react-app from GitHub and convert packages/react-scripts to a scoped package. For example, change this:

  "name": "react-scripts",
  "version": "0.7.0",

to this:

  "name": "@tabrezm/react-scripts",
  "version": "0.0.0",

3. Run npm link from packages/react-scripts.
4. Run npm link @tabrezm/react-scripts from the folder for the package created in step 1. Also, change the following lines in package.json from this:

  "devDependencies": {
    "react-scripts": "0.7.0"
  },

to this

  "devDependencies": {
    "@tabrezm/react-scripts": "0.0.0"
  },

5. Change something in App.js and run npm start.

Expected: see new content Actual: see content from template in react-scripts instead

Full discussion: https://github.com/facebookincubator/create-react-app/issues/682#issuecomment-263500172