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/packed_data_container.cpp

@@ -30,8 +30,8 @@
 
 #include "packed_data_container.h"
 
-#include "core_string_names.h"
-#include "io/marshalls.h"
+#include "core/core_string_names.h"
+#include "core/io/marshalls.h"
 
 Variant PackedDataContainer::getvar(const Variant &p_key, bool *r_valid) const {