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/io/pck_packer.cpp

@@ -29,8 +29,9 @@
 /*************************************************************************/
 
 #include "pck_packer.h"
+
 #include "core/os/file_access.h"
-#include "version.h"
+#include "core/version.h"
 
 static uint64_t _align(uint64_t p_n, int p_alignment) {