Fix importing npm linked libraries

Created by: AsaAyers

Looking through the npm link issues, I see two use cases:

1. (#1356) using npm link to test react-scripts
2. using npm link for libraries your app depends on.

This PR solves the 2nd case. Webpack's official solution for npm link is to add your project's node_modules to resolve.fallback on the config.

To verify the issue and fix create a small library to link:

cd /tmp
git clone https://gist.github.com/f40e1bad454626691097b4cc4651208c.git link-example
cd link-example
npm link
# to clean this up when you're done, return and run `npm unlink`

Then create your app and link the library.

create-react-app my-app
cd my-app
npm link link-example
echo 'import "link-example"' > src/index.js
npm run build

With the current published version, the build will fail:

> my-app@0.1.0 build /tmp/my-app
> react-scripts build

Creating an optimized production build...
Failed to compile.

Module not found: Error: Cannot resolve module 'react' in /tmp/link-example

Even though my-app imported ./node_modules/link-example/index.js, the symlink causes webpack to treat it like you required /tmp/link-example/index.js.