guix system: Simplify closure copy.

* guix/scripts/system.scm (copy-item): Add 'references' argument and
remove 'references*' call.  Turn into a non-monadic procedure.
(copy-closure): Remove initial call to 'references*'.  Only pass ITEM to
'topologically-sorted*' since that's equivalent.  Compute the list of
references corresponding to TO-COPY and pass it to 'copy-item'.

guix/scripts/system.scm

@@ -107,10 +107,10 @@ BODY..., and restore them."
   (store-lift topologically-sorted))
 
 
-(define* (copy-item item target
+(define* (copy-item item references target
                     #:key (log-port (current-error-port)))
-  "Copy ITEM to the store under root directory TARGET and register it."
-  (mlet* %store-monad ((refs (references* item)))
+  "Copy ITEM to the store under root directory TARGET and register it with
+REFERENCES as its set of references."
   (let ((dest  (string-append target item))
         (state (string-append target "/var/guix")))
     (format log-port "copying '~a'...~%" item)
@@ -131,23 +131,20 @@ BODY..., and restore them."
     (unless (register-path item
                            #:prefix target
                            #:state-directory state
-                             #:references refs)
+                           #:references references)
       (leave (G_ "failed to register '~a' under '~a'~%")
-               item target))
-
-      (return #t))))
+             item target))))
 
 (define* (copy-closure item target
                        #:key (log-port (current-error-port)))
   "Copy ITEM and all its dependencies to the store under root directory
 TARGET, and register them."
-  (mlet* %store-monad ((refs    (references* item))
-                       (to-copy (topologically-sorted*
-                                 (delete-duplicates (cons item refs)
-                                                    string=?))))
-    (sequence %store-monad
-              (map (cut copy-item <> target #:log-port log-port)
-                   to-copy))))
+  (mlet* %store-monad ((to-copy (topologically-sorted* (list item)))
+                       (refs    (mapm %store-monad references* to-copy)))
+    (for-each (cut copy-item <> <> target #:log-port log-port)
+              to-copy refs)
+
+    (return *unspecified*)))
 
 (define* (install-bootloader installer-drv
                              #:key