services: configuration: Generalize 'list-of-packages?'

* gnu/services/configuration.scm (list-of-packages?): Allow as well
tuples of a package and its output.
* gnu/home/services/sway.scm (list-of-packages): Remove duplicate.

Change-Id: I42622a29ac808c76928e9056dee9330d5e1b3f39
Signed-off-by: Ludovic Courtès <ludo@gnu.org>
Merges: #5470

gnu/home/services/sway.scm

@@ -129,9 +129,6 @@
 (define (list-of-string-ish? lst)
   (every string-ish? lst))
 
-(define (list-of-packages? lst)
-  (every package? lst))
-
 (define (bar-position? p)
   (member p '(top bottom)))
 

gnu/services/configuration.scm

@@ -535,7 +535,10 @@ the list result in @code{#t} when applying PRED? on them."
           #f)))
 
 (define list-of-packages?
-  (list-of package?))
+  (list-of (match-lambda
+             ((? package? pkg) #t)
+             (((? package? pkg) output) #t)
+             (_ #f))))
 
 (define list-of-strings?
   (list-of string?))