How to ignore `external` module style files

[qmhc]

Thanks for starting this plugin.

Now I have some use cases like this:

index.ts

import 'vexip-ui/dist/vexip-ui.css'
import './style.scss'

// `vexip-ui` is a node module

rollup.config.js

export default [
  {
    external: []
  },
  {
    external: ['vexip-ui']
  }
]

I would like to exclude the style of the module vexip-ui, but all bundles will include the external module style file.

Maybe same to #156 .