Move group-to-alist to the model utils module

So it can be more widely used.

guix-data-service/comparison.scm

@@ -24,20 +24,6 @@
 
             channel-news-differences-data))
 
-(define (group-to-alist process lst)
-  (fold (lambda (element result)
-          (match (process element)
-            ((key . value)
-             (match (assoc key result)
-               ((_ . existing-values)
-                `((,key . ,(cons value existing-values))
-                  ,@result))
-               (#f
-                `((,key . (,value))
-                  ,@result))))))
-        '()
-        lst))
-
 (define (derivation-differences-data conn
                                      base-derivation-file-name
                                      target-derivation-file-name)

guix-data-service/model/utils.scm

@@ -14,6 +14,7 @@
             parse-postgresql-array-string
             deduplicate-strings
             group-list-by-first-n-fields
+            group-to-alist
             insert-missing-data-and-return-all-ids))
 
 (define NULL '())
@@ -81,6 +82,20 @@
         '()
         lists))
 
+(define (group-to-alist process lst)
+  (fold (lambda (element result)
+          (match (process element)
+            ((key . value)
+             (match (assoc key result)
+               ((_ . existing-values)
+                `((,key . ,(cons value existing-values))
+                  ,@result))
+               (#f
+                `((,key . (,value))
+                  ,@result))))))
+        '()
+        lst))
+
 (define* (insert-missing-data-and-return-all-ids
           conn
           table-name