Currently exportReferencesGraph doesn't contain all information needed by nix-store --load-db in order to register paths as valid.

We use following hack, which is IO intense when generating a disk image:
https://github.com/NixOS/nixpkgs/blob/master/nixos/lib/make-disk-image.nix#L74-L76

What's the scope of this to be finished? @edolstra