Fixed plugin loaders trying to load plugins they aren't able to load

closes #2125

This is an API break for things implementing the PluginLoader interface.

src/pocketmine/plugin/PluginManager.php

@@ -210,6 +210,9 @@ class PluginManager{
 						continue;
 					}
 					$file = $directory . $file;
+					if(!$loader->canLoadPlugin($file)){
+						continue;
+					}
 					try{
 						$description = $loader->getPluginDescription($file);
 						if($description instanceof PluginDescription){