Fix output of pkg.list_repo_pkgs (yumpkg)

This allows for the full list of available packages in each repo to be
returned. This was my original intention when I wrote this function, but
I didn't realize that I was missing versions in the output until I
researched github issue #16467.

The structure of the return dict had to be changed slightly to
accommodate the new behavior.

Additionally, rather than just sorting each list in place, this sorts
all versions for a given package in a given repo using
distuils.version.LooseVersion, to give an accurate sorting of versions,
and put the newest versions at the top of the list.
Merge pull request #16503 from terminalmage/list-repo-pkgs

Fix output of pkg.list_repo_pkgs (yumpkg)