Avoid direct access to PHP files in plugin folder

Using our own versions of `async-uploads.php` can cause the plugin to not work on servers where access to PHP files directly is forbidden.

can we instead use the same approach as the network media library plugin?
```
add_action( 'load-async-upload.php', __NAMESPACE__ . '\switch_to_media_site', 0 );
add_action( 'wp_ajax_upload-attachment', __NAMESPACE__ . '\switch_to_media_site', 0 );
```