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/node_path.cpp

@@ -30,7 +30,7 @@
 
 #include "node_path.h"
 
-#include "print_string.h"
+#include "core/print_string.h"
 
 void NodePath::_update_hash_cache() const {