How to find a package file from substitute servers

[Pan Xie <xiepan@skyguard.com.cn>]

Hello

I ask this question because I encounter a very mundane problem: I want
to use the `at' command on my GuixSD system.

Like what I always do on Archlinux, the first step is to find out which
package includes the `at' command:

#+begin_example
pacman -Fx bin/at$
usr/bin/at is owned by extra/at 3.2.5-2
#+end_example

Then I'll install the package, and so on.

Unfortunately, I can't find a way to do the similar thing on GuixSD. I
know there is a “guix locate” to find files from store items, but
those store items must be already present in my gnu store.

So how to query such informations from a substitute server? Will Guix
plan to add a package file database in the future?

Thanks

Pan