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 is adapted from the `services.mopidy` NixOS module. The
difference is the setting can be configured with Nix language, taking
advantage of generators from nixpkgs. The module is also suited more
for user-specific configuration, removing the `extraConfigFiles` and
`dataDir` option.
