Upstream the listing of nix evaluation dependencies to nix

[Profpatsch]

lorri currently magically instrumentalizes every nix build it runs so that it can read the transitive dependency of all required nix source files and add them to its file watcher. This is brittle, as seen in #58.

Nix should instead be able to write structured information of which files it reads at evaluation, e.g. with an option like --write-evaluation-info=<output-file> or similar.

There is multiple use-cases for such an option, one is lorri of course, but another could be #64. The nix bazel integration, rules_nixpkgs does something similar as lorri to find out which files nix needs for evaluation and achieve better hermeticity.

[xzfc]

Related: someone started implementing nix-build --record a few years ago, but the PR was not got merged.
NixOS/nix#709