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

[Andrew Tropin]

describe-package and list-packages do not show emacs packages, installed
with guix. Packages themselves work perfectly fine, but not listed in
list-packages and can't be accessed with describe-package.

Way to reproduce:

guix environment --pure --ad-hoc emacs emacs-treemacs

emacs -q

M-x treemacs ;; Works fine
C-h P treemacs ;; Doesn't work
M-x list-packages ;; Doesn't list treemacs

Played around with it a little bit, still not sure how to solve.