Can’t install if wp-content/uploads in not empty

Trying to run `composer install` on an existing project gives error and fails :

```
In Filesystem.php line 314:
Could not delete wordpress/wp-content/uploads: 
Error: composer.phar install failed. Please check the lines above to find out why
```

I would expect that an existing `uploads` folder should be OK and not prevent the whole app from installing …