paths: use `list_paths` in `hg paths`
Using common code will make it simpler to update the logic behind the path
definition and storage.
Differential Revision: https://phab.mercurial-scm.org/D10441
urlutil: introduce a new `list_paths` function
This function will be useful for command and template that wants to display path
related information.
Differential Revision: https://phab.mercurial-scm.org/D10440