Reformat repository using new nixfmt-rfc-style.
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.
When smtps is used as a protocol, neomutt expects TLS but will if STARTTLS should be used. When using STARTTLS, smtp has to be used as protocol and `ssl_force_tls` is set. See <https://neomutt.org/guide/optionalfeatures#2-1-%C2%A0starttls>.