bug#70153: Command "guix show PACKAGE" should also show installed version

[Adam Porter <adam@alphapapa.net>]

Hi,

It would be very helpful if the command "guix show PACKAGE" (aka "guix 
package --show=PACKAGE") showed the version of the package that is 
currently installed, if any.

For example, the version of "emacs" in Guix is currently 29.3, and the 
version I have installed is 29.1.  When I run "guix show emacs" I get:

   name: emacs
   version: 29.3
   outputs: ...

If I didn't know better, I might think that I had version 29.3 
installed.  Instead, to check what version is actually installed, I have 
to run "guix package --list-installed=emacs" to see:

   emacs   29.1    out 
/gnu/store/9i6p8jsvgrsradmbqa7iixk61gd00k2g-emacs-29.1

This makes the common operation of comparing the currently installed 
version of a package with the currently available version require two 
commands instead of one.

In contrast, e.g. in Debian, one can run "apt-cache policy PACKAGE" to 
see output like:

   PACKAGE:
     Installed: X.Y-1~deb11u1
     Candidate: X.Y-1~deb11u1

Which makes it easy to compare them.

Thanks for your work on Guix.

--Adam