Instead of having to manually stub packages that should not be
downloaded we instead automatically stub all packages (except a small
list of whitelisted ones). Tests can re-introduce the real package by
using the `realPkgs` module argument.
This option provides a more convenient way to overlay dummy packages.
It also adds a function `config.lib.test.mkStubPackage` that can,
e.g., be used for `package` options.
`nix-index` is a tool to quickly locate the package providing a certain
file in `nixpkgs`. It indexes built derivations found in binary caches.
This module adds the shell integration for its `command-not-found`
script for interactive shells.
