Extensions - Setup <classloader> during "enable" and "uninstall"

This is an analog/follow-up to civicrm#20091

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

1. Enable extension $x
2. Disable extension $x
3. Either:
   * (a) Re-enable extension $x
   * (b) Uninstall extension $x

Step `#2` disables the classloader for purposes of regular page-loading. However, when you
get to step `#3a` or `#3b`, then you need the classloader again.

CRM/Extension/Manager/Module.php

@@ -29,7 +29,7 @@ public function __construct(CRM_Extension_Mapper $mapper) {
    * @param CRM_Extension_Info $info
    */
   public function onPreInstall(CRM_Extension_Info $info) {
-    CRM_Extension_System::singleton()->getClassLoader()->installExtension($info, dirname($this->mapper->keyToPath($info->key)));
+    $this->registerClassloader($info);
     $this->callHook($info, 'install');
     $this->callHook($info, 'enable');
   }
@@ -71,6 +71,7 @@ private function callHook(CRM_Extension_Info $info, $hookName) {
    * @return bool
    */
   public function onPreUninstall(CRM_Extension_Info $info) {
+    $this->registerClassloader($info);
     $this->callHook($info, 'uninstall');
     return TRUE;
   }
@@ -92,7 +93,15 @@ public function onPreDisable(CRM_Extension_Info $info) {
    * @param CRM_Extension_Info $info
    */
   public function onPreEnable(CRM_Extension_Info $info) {
+    $this->registerClassloader($info);
     $this->callHook($info, 'enable');
   }
 
+  /**
+   * @param CRM_Extension_Info $info
+   */
+  private function registerClassloader($info) {
+    CRM_Extension_System::singleton()->getClassLoader()->installExtension($info, dirname($this->mapper->keyToPath($info->key)));
+  }
+
 }