Change behavior of apt._clean_pkglist

The current behavior of this function will remove all instances of '1'
from the version list for a package. If the list only contains multiple
instances of '1', this will clear the list though, and that's not what
is wanted. The intention I had for this function when I wrote it was to
strip all virtual package markers if that package is both a real package
and virtual package, but also to take instances where more than one
target of a virtual package is installed, and make it so that only one
instance of '1' is in the version list for that package. This commit
adjusts the behavior to work that way.