Fix calling the same helper for separate packages in the override of dh binary-indep...

Fix calling the same helper for separate packages in the override of dh binary-indep/binary-arch. Closes: #520567

This is based on some work by Modestas Vainius, somewhat simplified
by a trick using excludes.

Note that the error in the case where there are no packages to build was
changed to a warning. That can easily happen now, and doesn't seem
particilarly error-worthy anyway; just exiting w/o doing anything seems
fine in that case.