profiles: 'check-for-collisions' filters out more non-collisions upfront.

When running:

  GUIX_PROFILING="gc object-cache" ./pre-inst-env \
    guix install gnome -p /tmp/sdsdfsd --no-grafts -n

this reduces the number of object cache lookup from 115K to 96K, with a
slight CPU and memory usage reduction as well.

* guix/profiles.scm (check-for-collisions)[candidates]: Exclude entries
if their 'item' fields are eq?.

guix/profiles.scm

@@ -334,7 +334,10 @@ file name."
     (filter-map (lambda (entry)
                   (let ((other (lookup (manifest-entry-name entry)
                                        (manifest-entry-output entry))))
-                    (and other (list entry other))))
+                    (and other
+                         (not (eq? (manifest-entry-item entry)
+                                   (manifest-entry-item other)))
+                         (list entry other))))
                 (manifest-transitive-entries manifest)))
 
   (define lower-pair