Feature request: option to include build inputs when pushing

[gmacon]

I use attic to cache the build results of my CI system, and I have a script that, after pushing the result, it gets the closure of the deriver of result and then pushes the outputs of those derivations that exist locally. I recently noticed that this doesn't actually include everything, since I also need to recurse through those things, too.

I would like an option to push everything necessary to rebuild the output as if only my cache and a copy of cache.nixos.org were available, which I think is the same as "everything that would be kept by a garbage collection if the arguments to push were gcroots and the keep-outputs and keep-derivations options are enabled. (And, nice to have, then with store derivations (.drv files) filtered out, since they are rebuilt during the evaluation process.)