libphonon-dev should depend on "phonon-backend-null | phonon-backend"?

Hi,

[ CCing libphonon-dev maintainers ]

while trying to fix random RC bugs, I stumbled over this one and tried 
to
reproduce it in my local cowbuilder setup (you used sbuild).
This seems not to happen on buildds, as they use sbuilds internal 
resolver.

cowbuilder uses aptitude to resolve Build-Dependencies and fails in the 
same way
your sbuild did:

After that I executed cowbuilder --login to debug further.
`aptitude install libphonon-dev phonon-backend-gstreamer` gives
 The following packages have unmet dependencies:
  phonon-backend-null : Conflicts: phonon-backend which is a virtual package.
 The following actions will resolve these dependencies:

      Keep the following packages at their current version:
 1)     libphonon-dev [Not Installed]
 2)     phonon-backend-null [Not Installed]
(cowbuilder aborts here).

However, `apt-get install libphonon-dev phonon-backend-gstreamer` works just
fine. Not sure why it tries phonon-backend-null at all (I requested
phonon-backend-gstreamer), but I think this is because libphonon-dev 
depends on
phonon-backend-null | phonon