Refuse to build if user is importing something that isn't declared in package.json

Created by: gaearon

I think Brunch does this, if I recall correctly.

This is an important feature because people often misunderstand that transitive dependencies aren’t supposed to be importable.

See https://github.com/facebookincubator/create-react-app/issues/1714 and https://github.com/facebookincubator/create-react-app/issues/1622 for examples.