As you can see, there are many odd files in the
"…/share/emacs/site-lisp/guix.d/mmm-mode-0.5.4" directory.

So I suggest to add a phase for deleting non-".el[c]" files from the
".guix.d/package" directory.

And just in case: I have nothing against GNU ELPA repository (especially
taking into account that it is the only "home" for some packages).  I'm
against melpa and melpa-stable, because:

- Why should we rely on a third-party server that do something with the
  upstream files to produce a final tarball?

- MELPA(-stable) is not usable anyway, because the tarballs of the same
  version are updated all the time, so the hash is being permanently
  changed.

However, if a package from ELPA has a real upstream release that can be
used with "gnu-build-system" (e.g., emms, auctex, mmm-mode), I think we
should prefer it instead of importing it from ELPA.

-- 
Alex