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/quick_open.cpp

@@ -30,7 +30,7 @@
 
 #include "quick_open.h"
 
-#include "os/keyboard.h"
+#include "core/os/keyboard.h"
 
 void EditorQuickOpen::popup(const StringName &p_base, bool p_enable_multi, bool p_add_dirs, bool p_dontclear) {