When a local account address is forwarded, the mails were not locally
kept. This was due to the way lookup tables were internally managed.
Instead of using lists to represent Postfix lookup tables, we now use
attribute sets: they can then be easily merged.
A regression test for
https://gitlab.com/simple-nixos-mailserver/nixos-mailserver/-/issues/
has been added: it sets a forward on a local address and ensure an
email sent to this address is locally kept.
Fixes#205
Martin Weinelt