Split the package silverpeas into two packages: silverpeas which provides the collaborative portal itself, and silverpeas-postgresql which configures Silverpeas to be used with PostgreSQL as datasource; latter provides the virtual package silverpeas-datasource on which depends the package silverpeas
It looks like dpkg-gencontrol does not use the control file specified with option -c for the lock-file (at least on my raring ringtail
Thus I copy the file in the expected debian/control place before running the dpkg-gencontrol command then I delete it.