guix package: '-s' sorts packages by name, then by version.

Before that it would sort them by name only, so the order in which two
packages with the same name but a different version would appear was
non-deterministic.

Reported by Tomáš Čech <sleep_walker@gnu.org>.

* guix/scripts/package.scm (find-packages-by-description)[version<?]:
  New variable.
  Change the 2nd argument to 'sort' to use 'string-compare' and resort
  to 'version<?' when P1 and P2 have the same name.

guix/scripts/package.scm

@@ -240,6 +240,8 @@ DURATION-RELATION with the current time."
 (define (find-packages-by-description rx)
   "Return the list of packages whose name, synopsis, or description matches
 RX."
+  (define version<? (negate version>=?))
+
   (sort
    (fold-packages (lambda (package result)
                     (define matches?
@@ -254,8 +256,11 @@ RX."
                         result))
                   '())
    (lambda (p1 p2)
-     (string<? (package-name p1)
-               (package-name p2)))))
+     (case (string-compare (package-name p1) (package-name p2)
+                           (const '<) (const '=) (const '>))
+       ((=)  (version<? (package-version p1) (package-version p2)))
+       ((<)  #t)
+       (else #f)))))
 
 (define-syntax-rule (leave-on-EPIPE exp ...)
   "Run EXP... in a context when EPIPE errors are caught and lead to 'exit'