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

I'm trying to pack a subset of antd using this plugin with rollup,  and here's my code in the entry:
``` ts
export { Button } from 'antd'
```
and no css file is emitted. I have to change my code as below to make it work :
``` ts
import { Button } from 'antd'
export { Button }
```
