Virtual packages

To make this easier to handle we use virtual packages.
This provides a method to state that a package provides
a certain interface or functionality. This is done by
introducing a provides-relation. 

Using this we now get:

     Package: mutt
     Depends: mail-transport-agent

     Package: sendmail
     Provides: mail-transport-agent

     Package: smail
     Provides: mail-transport-agent