[packaging] hard dependency on mysql backend, breaks other programs

Kubuntu 12.04 x86_64. Kubuntu PPAs added.

Akonadi has a hard dependency on akonadi-backend-mysql despite there being other backends available. By extension, this means that mysql-{server,client}-core-5.1 needs to be installed to get a working plasma workspace. Please see LP bug 923189 for some more information about this decision;
> https://bugs.launchpad.net/ubuntu/+source/akonadi/+bug/923189

There are MySQL drop-in replacements (such as Percona; https://launchpad.net/percona-server - admittedly not in our repos) that conflict/replace the mysql packages. Our akonadi packaging makes KDE mutually exclusive with those, even though the necessary backend functionality may be attained by installing a different akonadi backend. Please see this thread on ubuntuforums.org;
> http://ubuntuforums.org/showthread.php?t=1960064

I'm not versed in the art of packaging, but two obvious solutions spring to mind. In reverse order of difficulty;

1. Influence the packaging of everything related to mysql (our own MySQL, Percona, equavilents) to provide a 'mysqld' virtual package, and have the implementations provide it in turn. It's a neat solution for sure but requires effort. However, akonadi-server would still depend on mysql (whatever flavor) and eat space.

2. Make akonadi-server depend on a virtual 'akonadi-backend' package and have all the backends provide it. Akonadi can recommend the (best supported default) mysql backend and suggest the others, but still depend on the virtual one so as to always enforce there being a backend installed. Most users won't be affected and will still just use the mysql one, and those with special requirements (eg "cannot/must not/don't want to install mysql") get alternatives.

To test the second solution, I built akonadi-server locally (source from precise main) after these quick modifications to debian/control. It built nicely and I'm now running with only akonadi-backend-sqlite installed. Please see the attached patch for an example. (Semi-related is that there's still a akonadi-backend-virtuoso virtual package that nothing seems to provide anymore?)

Worthy of consideration here is KDE bug 298314 that makes it impossible to actually change to an akonadi backend other than mysql and postgresql (via the akonadi kcm). Those who opt out of akonadi-backend-{my,postgre}sql will be greeted with an Akonadi crash upon login unless they manually edit ~/.config/akonadi/akonadiserverrc and specify the backend to use there.
> https://bugs.kde.org/show_bug.cgi?id=298314