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_scale.cpp

@@ -30,7 +30,7 @@
 
 #include "editor_scale.h"
 
-#include "os/os.h"
+#include "core/os/os.h"
 
 static float scale = 1.0;