Arguably, this is a bug in Nix's structuredAttrs: without
structuredAttrs, exportReferencesGraph with an empty path set would
still result in information being provided. With structuredAttrs, no
info is provided for an empty path set.
Nevertheless, we need to be able to build even if Nix has the bug, so
work around it by checking for an empty path set and handling it
explicitly.
Derivations affected by this patch set `__structuredAttrs = true;` and
provide their own `builder`, i.e. it's necessary to `source .attrs.sh`.
Rather than adding even more `if`-`source` monstrums, I decided to
modify all of those derivations to use `buildCommand` or `runCommand`,
without `builder` being set.
Then, `$stdenv/setup` is sourced already and as a result it's safe to
assume that `NIX_ATTRS_JSON_FILE`/`NIX_ATTRS_SH_FILE` point to a usable
location both in a build and a shell session.
The previous one was very bad and worsened the situation.
But even running with some nix-1.12 I'm unable to reproduce
the original failure. Let's unblock channels for now.
Unlike pathsFromGraph, on Nix 1.12, this function produces a
registration file containing correct NAR hash/size information.
https://hydra.nixos.org/build/62832723
