size: Fix corner case with multiple items on the command line.

Fixes 'guix size foo bar' when 'foo' is in the local store and 'bar'
isn't, which could lead to an incomplete requisite list.

* guix/scripts/size.scm (requisites*): Partition ITEMS according to
'valid-path?'.  Use 'substitutable-requisites' only on invalid items.

guix/scripts/size.scm

@@ -29,6 +29,7 @@
   #:use-module (srfi srfi-1)
   #:use-module (srfi srfi-9)
   #:use-module (srfi srfi-11)
+  #:use-module (srfi srfi-26)
   #:use-module (srfi srfi-34)
   #:use-module (srfi srfi-37)
   #:use-module (ice-9 match)
@@ -119,10 +120,12 @@ substitutes."
 information available in the local store or using information about
 substitutes."
   (lambda (store)
-    (guard (c ((nix-protocol-error? c)
-               (values (substitutable-requisites store items)
-                       store)))
-      (values (requisites store items) store))))
+    (let-values (((local missing)
+                  (partition (cut valid-path? store <>) items)))
+      (values (delete-duplicates
+               (append (requisites store local)
+                       (substitutable-requisites store missing)))
+              store))))
 
 (define (store-profile items)
   "Return as a monadic value a list of <profile> objects representing the