Core: Move DirAccess and FileAccess to core/io

File handling APIs are typically considered part of I/O, and we did have most
`FileAccess` implementations in `core/io` already.

core/io/pck_packer.cpp

@@ -31,9 +31,9 @@
 #include "pck_packer.h"
 
 #include "core/crypto/crypto_core.h"
+#include "core/io/file_access.h"
 #include "core/io/file_access_encrypted.h"
 #include "core/io/file_access_pack.h" // PACK_HEADER_MAGIC, PACK_FORMAT_VERSION
-#include "core/os/file_access.h"
 #include "core/version.h"
 
 static int _get_pad(int p_alignment, int p_n) {