suggest transport-packages based on established namescheme

suggest transport-packages based on established namescheme

apt-transports not shipped in apt directly are usually named
apt-transport-% with % being what is in the name of the transport.
tor additional introduced aliases via %+something, which isn't a bad
idea, so be strip the +something part from the method name before
suggesting the installation of an apt-transport-% package.

This avoids us the maintainance of a list of existing transports
creating a two class system of known and unknown transports which would
be quite arbitrary and is unfriendly to backports.