Extensions - Setup <classloader> during un-installation

This is a somewhat counter-intuitive analog to civicrm#20091
which allows you to use classes during the uninstall procedure.

Why is this necessary? Recall that a typical admin will go through this lifecycle:

1. Enable extension $x
2. Disable extension $x
3. Uninstall extension $x

Step `#2` disables the classloader for purposes of regular page-loading. However, in step `civicrm#3`,
you need the classloader again

CRM/Extension/Manager/Module.php

@@ -71,6 +71,7 @@ private function callHook(CRM_Extension_Info $info, $hookName) {
    * @return bool
    */
   public function onPreUninstall(CRM_Extension_Info $info) {
+    CRM_Extension_System::singleton()->getClassLoader()->installExtension($info, dirname($this->mapper->keyToPath($info->key)));
     $this->callHook($info, 'uninstall');
     return TRUE;
   }