to do this with a `wget -r' but the manifest contains absolute
rather than relative URLs, so those have to be rewritten. And since
they have to be rewritten anyway, we can add size and hash fields to
make the manifest compatible with older version of Nix (and make the
download size estimate work).
git-svn-id: https://nixos.org/repos/nix/release/trunk/channels@15076 70bd8c7a-acb8-0310-9f0d-9cc1c95dcdbb