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/PluginLoader.php

@@ -53,6 +53,15 @@ interface PluginLoader{
 	 */
 	public function getPluginFilters() : string;
 
+	/**
+	 * Returns whether this PluginLoader can load the plugin in the given path.
+	 *
+	 * @param string $path
+	 *
+	 * @return bool
+	 */
+	public function canLoadPlugin(string $path) : bool;
+
 	/**
 	 * @param Plugin $plugin
 	 *