MDL-49329 admin: Call opcache_reset() after installing new plugin code

During the testing, I was experiencing weird behaviour - after a single
plugin updated was installed, I ended up on admin/index.php?cache=1. I
believe it was caused by the missing opcache_reset().

lib/classes/plugin_manager.php

@@ -1321,10 +1321,16 @@ public function install_remote_plugins(array $plugins, $confirmed, $silent) {
             if (!$this->unzip_plugin_file($zipfile, $target, $pluginname)) {
                 $silent or $this->mtrace(get_string('error'));
                 $silent or $this->mtrace('Unable to unzip '.$zipfile, PHP_EOL, DEBUG_DEVELOPER);
+                if (function_exists('opcache_reset')) {
+                    opcache_reset();
+                }
                 return false;
             }
             $silent or $this->mtrace($ok);
         }
+        if (function_exists('opcache_reset')) {
+            opcache_reset();
+        }
 
         return true;
     }