Allow using package path for copyFiles()

The bundle will register a path like this 

```json
"@symfony/ux-turbo": "file:vendor/symfony/ux-turbo/Resources/assets",
```

It would be nice to be able to use this in the `copyFiles()`, like so

```javascript
Encore.
    .copyFiles({
        from: '@symfony/ux-turbo/css',
        to: 'css/[path][name].[hash:8].[ext]',
    });
```