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.

main/tests/test_gdscript.cpp

@@ -30,9 +30,9 @@
 
 #include "test_gdscript.h"
 
-#include "os/file_access.h"
-#include "os/main_loop.h"
-#include "os/os.h"
+#include "core/os/file_access.h"
+#include "core/os/main_loop.h"
+#include "core/os/os.h"
 
 #ifdef GDSCRIPT_ENABLED