tracking fods

[Mic92]

For archiving sources it would be good to trace FODs in nix-eval-jobs and for testing those for reproducibility. Since those might be not available through attributes we might need to access them through derivations. We should log all FODs in the entire tree.

[adisbladis]

I think that this feature should be implemented as a generic "stream all intermediate derivations" mode instead of just FODs.
This allows for even more use cases, and it's pretty easy to filter the stream for FODs.