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.
The `pkgs.gnome.gnome-terminal` package was moved to
`pkgs.gnome-terminal`. The former is now a deprecated alias that
throws a warning whenever a configuration enabling the module is used:
```
The ‘gnome.gnome-terminal’ was moved to top-level. Please use
‘pkgs.gnome-terminal’ directly.
```
Related: https://github.com/NixOS/nixpkgs/pull/319659
Related: https://github.com/nix-community/home-manager/pull/5611
