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.
Neomutt will run the given command (which can be a string or a path)
and take the output from stdout and use it as the signature for your
email.
Co-authored-by: Nicolas Berbiche <nicolas@normie.dev>
