Refactor `...-get-show-packages' functions

`guix-list-get-show-packages'/`guix-info-get-show-packages' show
information in list/info buffers unconditionally; and the new
`guix-get-show-packages' function takes care of
`guix-list-single-package' variable.
Support information about installed packages

The "engine" for displaying info is improved.  Many small modifications.
Add/fix guix paths