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/file_access_encrypted.cpp

@@ -30,9 +30,9 @@
 
 #include "file_access_encrypted.h"
 
+#include "core/os/copymem.h"
+#include "core/print_string.h"
 #include "core/variant.h"
-#include "os/copymem.h"
-#include "print_string.h"
 
 #include "thirdparty/misc/aes256.h"
 #include "thirdparty/misc/md5.h"