Make it a proper multipackage monorepo

Created by: gaearon

I think we should change the directory structure to make it easier to add more packages to the project in the future. I’m thinking something like this:

packages
  - create-react-app (this is global-cli in current layout)
    - index.js
    - package.json
  - react-scripts (this is project root in current layout)
    - bin
    - config
    - scripts
    - package.json
tasks
template

A PR doing this is welcome. (You’ll probably want to adjust config/paths.js to account for this change.)

If you plan to work on this please leave a comment here so multiple people don’t work on the same thing. Please respect it if somebody has taken the task, and let them finish it.