replaced class_exists to interface_exists for interfaces

src/Plugin/WithoutNamespace/Plugin_Info.php

@@ -3,10 +3,10 @@
 if ( ! interface_exists( 'WPDesk_Translatable' ) ) {
 	require_once __DIR__ . '/Translatable.php';
 }
-if ( ! class_exists( 'WPDesk_Buildable' ) ) {
+if ( ! interface_exists( 'WPDesk_Buildable' ) ) {
 	require_once __DIR__ . '/Buildable.php';
 }
-if ( ! class_exists( 'WPDesk_Has_Plugin_Info' ) ) {
+if ( ! interface_exists( 'WPDesk_Has_Plugin_Info' ) ) {
 	require_once __DIR__ . '/Has_Plugin_Info.php';
 }