mail: fix dependency cycle

There is a dependency cycle right now with
 (Class[Exim4] => Class[Exim::Roled::Mailman] =>
 Exim4::Dkim[lists.wikimedia.org] =>
 File[/etc/exim4/dkim/lists.wikimedia.org-wikimedia.key] =>
 Service[exim4] => Class[Exim4])

This is a byproduct of managing aliases & domainlists manually,
something that will be fixed in a subsequent iteration.

Fix this by removing class dependencies and replacing them with more
granular require => Class['exim4'] dependencies.

Change-Id: I7687c9fa8571b7089037a4b36cba34164c0718b9