Add versions_as_list kwarg to version, list_pkgs

This commit adds the ability to optionally get the version information
as a list of strings, as opposed to a string. All package providers have
been updated with this (even though very few providers actually support
multiple versions of the same package), so as not to break pkg states.

Reference: https://github.com/saltstack/salt/issues/2783#issuecomment-14011820