apt: fix pkg.latest_version

If a particular package name is both the name of a package and also the
name of a virtual package, the version comparison used to determine if
the install candidate is an upgrade fails. This commit fixes #5455 by
stripping the virtual package markers from the package list returned by
list_pkgs().
Merge pull request #5472 from terminalmage/issue5455

apt: fix pkg.latest_version