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/plugins/sprite_frames_editor_plugin.cpp

@@ -30,9 +30,9 @@
 
 #include "sprite_frames_editor_plugin.h"
 
+#include "core/io/resource_loader.h"
+#include "core/project_settings.h"
 #include "editor/editor_settings.h"
-#include "io/resource_loader.h"
-#include "project_settings.h"
 #include "scene/3d/sprite_3d.h"
 
 void SpriteFramesEditor::_gui_input(Ref<InputEvent> p_event) {