This PR is part of the networking.* namespace cleanup.
ssmtp used to be configured via `networking.defaultMailServer` which is
sort of misleading since it provides options only for ssmtp. Other
dumb mail relays like nullmailer have always been living under
services.
The intent of this PR is to align ssmtp's options with those of similar
services. Specifically, two renames have been done:
* Rename `networking.defaultMailHost` to `services.ssmtp`.
* Rename `directDelivery` to `enable` because this is what it basically does.
Using pkgs.lib on the spine of module evaluation is problematic
because the pkgs argument depends on the result of module
evaluation. To prevent an infinite recursion, pkgs and some of the
modules are evaluated twice, which is inefficient. Using ‘with lib’
prevents this problem.
