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.

core/os/main_loop.cpp

@@ -29,7 +29,8 @@
 /*************************************************************************/
 
 #include "main_loop.h"
-#include "script_language.h"
+
+#include "core/script_language.h"
 
 void MainLoop::_bind_methods() {