nixpkgs

Is your feature request related to a problem? Please describe.
using rules_node with the nixpkgs node is extremely painful and requires the workaround specified in bazelbuild/rules_nodejs#464 / bazelbuild/bazel#2927

Specifically:

nixpkgs_package(
    name = "nixpkgs_nodejs",
    build_file_content = 'exports_files(glob(["nixpkgs

It would be convenient if we could ship some bash, zsh, and fish shell completions for the cargo2nix code generation tool. We could maintain these shell completions ourselves, or ideally, we could switch to clap and/or structopt for argument parsing and generate these completions at build-time with a build.rs.