Hello,

The following opam package import fails

Command: `guix import opam -r mirage-crypto-pk`

Output:

```

Starting download of /tmp/guix-file.ZERar3
From https://github.com/mirage/mirage-crypto/releases/download/v0.10.6/mirage-crypto-0.10.6.tbz...
following redirection to `https://objects.githubusercontent.com/github-production-release-asset-2e65be/238478536/d79d53ac-d704-42c2-a899-b487d768b5f1?X-Amz-Algorithm=AWS4-HMAC-SHA256&X-Amz-Credential=AKIAIWNJYAX4CSVEH53A%2F20220727%2Fus-east-1%2Fs3%2Faws4_request&X-Amz-Date=20220727T212413Z&X-Amz-Expires=300&X-Amz-Signature=b17374c9f77069f49807d91742bc1fd32c5676b916e3b801b695a54f8398dcd7&X-Amz-SignedHeaders=host&actor_id=0&key_id=0&repo_id=238478536&response-content-disposition=attachment%3B%20filename%3Dmirage-crypto-0.10.6.tbz&response-content-type=application%2Foctet-stream'...
 …o-0.10.6.tbz  1.2MiB                8.3MiB/s 00:00 [##################] 100.0%
Backtrace:
          10 (primitive-load "$HOME/.config/guix/current/bin/gu…")
In guix/ui.scm:
   2238:7  9 (run-guix . _)
  2201:10  8 (run-guix-command _ . _)
In guix/scripts/import.scm:
    92:11  7 (guix-import . _)
In guix/scripts/import/opam.scm:
   103:17  6 (guix-import-opam . _)
In guix/import/utils.scm:
   539:27  5 (recursive-import _ #:repo->guix-package _ #:guix-name _ …)
   529:33  4 (lookup-node "mirage-crypto-pk" #f)
In guix/import/opam.scm:
    284:2  3 (opam->guix-package "mirage-crypto-pk" #:repo _ # _)
In unknown file:
           2 (filter #<procedure 7fd67cb02ce0 at guix/import/opam.s…> …)
In guix/import/opam.scm:
   287:13  1 (_ ("mirage-no-solo5" "mirage-no-xen"))
In unknown file:
           0 (string-prefix? "conf-" ("mirage-no-solo5" "mirage-n…") …)

ERROR: In procedure string-prefix?:
In procedure string-prefix?: Wrong type argument in position 2 (expecting string): ("mirage-no-solo5" "mirage-no-xen")

```