Subject: Package outputs should be described in the UI

From: Leo Famulari <[email-masked]>
Date: Tue, 20 Apr 2021 13:38:31 -0400

At least once a month I have to help somebody on IRC who didn't notice
that our Git package is split into multiple outputs.

They can't find `git send-email`, and the Guix package UI doesn't make
this clear enough.

I had the idea that package outputs besides 'out' and 'debug' should
have very brief descriptions that are exposed in `guix show` and `guix
search` and the `guix package` equivalents.

So, maybe it would look like this:

------
$ guix show git
name: git
version: 2.31.1
outputs: out send-email svn credential-netrc credential-libsecret subtree gui
  * send-email: Tools for an email-based Git workflow
  * svn: Provides git-svn, for bidirectional operation between Subversion and Git repositories
  * credential-libsecret: Whatever that is
  * subtree: Who knows?
  * gui: A graphical interface for exploring a Git repository
  * etc...
systems: x86_64-linux i686-linux
dependencies: [email-masked] [email-masked] [email-masked] [email-masked] [email-masked] [email-masked] [email-masked] [email-masked] [email-masked] [email-masked] [email-masked]
+ [email-masked] [email-masked] [email-masked] [email-masked] [email-masked] [email-masked] [email-masked] [email-masked] [email-masked] [email-masked] [email-masked]
+ [email-masked] [email-masked]
location: gnu/packages/version-control.scm:173:2
homepage: https://git-scm.com/
license: GPL 2
synopsis: Distributed version control system  
description: Git is a free distributed version control system designed to handle everything from small to very large projects with speed and efficiency.
------

The output of `guix show` and `guix search` are in recutils format, and
we'd need to make sure that these new UI elements continued that
valuable tradition.