> (Also, the manual annotation would go away once 'gs-fonts' is renamed.
> I'll run "echo /gnu/store/*/share/fonts > atchie-fonts" though to see
> if there are other packages not following the convention.)

Scheme code:

(use-modules (ice-9 ftw) (srfi srfi-1))
(define (has-share/fonts? o)
  (file-exists? (string-append "/gnu/store/" o "/share/fonts")))
(define t (scandir "/gnu/store" has-share/fonts?))
(define t2 (map (lambda (x) (substring x 33)) t))
(define t3 (delete-duplicates t2))
(define (fonty? x) (string-prefix? "font-" x))
(partition (lambda (x) (string-prefix? "font-" x)) t3)

Output on my system:

$1 = ("font-alias-1.0.3" [...])
$2 = ("fonts-dir" "profile" "mate-1.24.1" "denemo-2.5.0" "mate-1.24.0" "gs-fonts-8.11")

"fonts-dir", "profile" and "gs-fonts" are expected.

"mate" uses "union-build" and has "font-cantarell" in 'inputs'.
Apparently "denemo" has some font files as well.
Something will need to be figured out for these cases,
if the approach of my patch is followed.

I think it's better to optimise 'union-build' and the store deduplication
code though (maybe subdirectories can be process concurrently, to maximise I/O
utilisation?). That should improve performance outside the 'fonts-dir-file' hook
as well.

Greetings,
Maxime.