Fix computing the list of unused modules by using the full `_zdirs`
instead of just the `_zmodules` names. The fixed code was even simpler
and cleaner, which makes it even more satisfying. :- )
Show "not found" instead of "not installed" error when an external
module dir does not exist, since we don't install external modules.
Using the `list` action with `-v` shows all current details for the
existing modules.
Don't try to install or update external modules, since they have an
absolute path instead of a valid URL.