> [Adding Arun Isaac to CC.  Their commit d8796851 is the first one to
> drop -pkg.el, but without explanation.]

Commit d8796851 was an attempt to not install too many unnecessary files
and be closer to how MELPA packaged emacs packages. See
https://lists.gnu.org/archive/html/guix-devel/2017-04/msg00274.html for
the detailed discussion.

Cheers!