yumpkg: rewrite pkg.list_repo_pkgs()

This function was added just before yumpkg was initially deprecated, and
when I ported it to yumpkg5 I enhanced it considerably. This commit
rewrites this function in yumpkg to make it functionally identical to
the yumpkg5 version.
Merge pull request #9819 from terminalmage/yum-refactoring

yumpkg: rewrite pkg.list_repo_pkgs()