Keep "import css" line to the output file?

# Rollup Config
```
export default {
input: "src/index.js",
output: {
file: "dist/index.js",
format: "cjs"
},
plugins: [
postcss({
extract: true
})
]
};
```

### src/index.js
```
import "./index.less";
```

# Expected bundle
```
require("./index.css")
```

- But the actual result is an empty bundle