bug#48331: Emacs' describe-package doesn't work for packages managed by guix

[Andrew Tropin <andrew@trop.in>]

> The problem here is, that Guix does not include the <package>-pkg.el
> file, that would typically be generated by package.el.  To deal with
> this, you have to provide package specs on your own.  There already
> exists a utility to locate libraries in a package manager agnostic
> fashion [1], all you need to do is to feed back that information to
> package.el.

Yep, I figured it out yesterday and even hacked around a little bit.

Patched emacs-build-system to place packages under elpa/NAME-VERSION
subdirectory and removed "-pkg\\.el$" from %default-exclude.

> I have now published emacs-dpd [2], which does exactly that.  To use it
> for your Guix Emacs packages, execute
>   (dpd (list "$GUIX_PROFILE/share/emacs/site-lisp" ...))
> replacing "$GUIX_PROFILE" with the actual profile, after `package-
> initialize' has run with `dpd-fuzzy-recognize' in `dpd-recognize-hook'.
> I might write a more detailed README later.

Most of the packages already have -pkg.el in sources, but yep, pretty
cool utility, also thought about implementing something like that
yesterday, but luckily I didn't and now I do not need to do it, because
now I'm aware of already-existing implementations!)

> Neither packed nor dpd are currently packaged in Guix.  packed can
> easily be imported from melpa-stable, but for dpd you'd have to write
> your own guix.scm.  I might do that at some point as well.

> We already had modifications in emacs-build-system recently, so if you
> want to argue, that all Emacs packages should have that <package>-
> pkg.el to work with package.el out of the box, I would ask you to wait,
> so as to not cause an "Emacs world" rebuild again after only ten days.
> I also don't know whether Guix has the same information as package.el
> at build time, but that might change with time as well.  Particularly,
> there will hopefully be a move towards supplying name and version at
> build, which would give us the most important information.

Very cool, I didn't have the latest changes on my local checkout and didn't
see your commits, but now I see, it is exactly what I needed.

The only side note: it should be site-lisp/elpa/NAME-VERSION (right now
it is site-lisp/NAME-VERSION).  Also, on line 137 elpa-directory
function can be used.

When you will be updating the path, please remove -pkg.el from
%default-exclude.

Thank you very much for your work!  Really appreciate it!

-- 
Best regards,
Andrew Tropin