zypper: fix pkg.latest_version

latest_version was using list_upgrades(), which in turn ran
'zypper list-updates'. This, however, does not give information for
packages which are available but are not presently installed. Changed
latest_version so that it parses the output from 'zypper info'.

Fixes #5166.
Merge pull request #5167 from terminalmage/issue5166

zypper: fix pkg.latest_version