The corresponding image path string does not exist in the css file, what should I do?

### Description

https://www.npmjs.com/package/flag-icon-css


#### Steps to reproduce


style.css
```
.flag-icon-ad {
    background-image: url(../flags/1x1/ad.svg);
}
```


rev-manifest.json
`  "flag-icon-css/flags/1x1/ad.svg": "flag-icon-css/flags/1x1/ad-e15ddeabbf.svg",
`

无法实现图片url的替换。
nodejs版本: ^12.0.0




