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.

modules/visual_script/visual_script.cpp

@@ -30,8 +30,8 @@
 
 #include "visual_script.h"
 
-#include "os/os.h"
-#include "project_settings.h"
+#include "core/os/os.h"
+#include "core/project_settings.h"
 #include "scene/main/node.h"
 #include "visual_script_nodes.h"