Add support for `export { sth } from 'lib'` case

[cyndra0]

I'm trying to pack a subset of antd using this plugin with rollup, and here's my code in the entry:

export { Button } from 'antd'

and no css file is emitted. I have to change my code as below to make it work :

import { Button } from 'antd'
export { Button }