Nested CSS mixin with relative paths do not resolve

```css
/* email.css */
input[type='email'] {
  @mixin input from url('/app/assets/theme/mixins/input.css');
}
```

```css
/* input.css */
@define-mixin input {
  @mixin field from url('./field.css');
  display: block;
}
```

Results in...

```css
input[type='email'] {
  @mixin field from url('./field.css');
  display: block;
}
```