Referenced images or other assets (via `url()`) are not moved (or URLs not updated) to be correct it output CSS

Input CSS:
```css
/* src/components/setup/command-bar/empty-state.css */

@layer myLayer {
.empty {
/* command-empty.png exists at src/components/setup/command-bar/command-empty.png */
background: url("./command-empty.png") no-repeat;
}
}
```

config:
```js
postcss({ extract: 'styles.css' }),
```

output:
```css
/* dist/styles.css */
@layer myLayer {
.empty {
/* there is no command-empty.png in dist */
background: url("./command-empty.png") no-repeat;
}
}
```

same issue as
- https://github.com/thgh/rollup-plugin-css-only/issues/64
- https://github.com/jleeson/rollup-plugin-import-css/issues/42