Make core/ includes absolute, remove subfolders from include path

This allows more consistency in the manner we include core headers,
where previously there would be a mix of absolute, relative and
include path-dependent includes.

editor/editor_data.cpp

@@ -30,12 +30,12 @@
 
 #include "editor_data.h"
 
+#include "core/io/resource_loader.h"
+#include "core/os/dir_access.h"
+#include "core/os/file_access.h"
+#include "core/project_settings.h"
 #include "editor_node.h"
 #include "editor_settings.h"
-#include "io/resource_loader.h"
-#include "os/dir_access.h"
-#include "os/file_access.h"
-#include "project_settings.h"
 #include "scene/resources/packed_scene.h"
 
 void EditorHistory::cleanup_history() {