don't read outputs into memory for output rewriting
don't read outputs into memory for output rewriting

RewritingSink can handle being fed input where a reference crosses a
chunk boundary. we don't need to load the whole source into memory, and
in fact *not* loading the whole source lets nix build FODs that do not
fit into memory (eg fetchurl'ing data files larger than system memory).

NixOS/nix,NixOS/nix,NixOS/nix,NixOS/nix,NixOS/nix