handle pkgnames shorter than modifiers

handle pkgnames shorter than modifiers

The bugreport highlights the problem with an empty package name. We fix
this by 'ignoring' these so that it behaves just like "apt-get install".
The deeper problem is that modifier strings can be longer than a package
name in which case the comparison doesn't make sense, so don't compare
then. Was not noticed so far as all modifiers are of length 1, so the
only package name shorter than this is in fact the empty package name.

Closes: 744940