bug#22693: `guix refresh -u` updates other packages with same version

[Leo Famulari]

I've noticed that `guix refresh -u` will update extraneous packages if
they happen to have the same version and be in the same module.

For example, from commit d694230ab, you can reproduce the bug:

$ ./pre-inst-env guix environment guix -- ./pre-inst-env guix refresh -u 
python-pytest
$ git diff
diff --git a/gnu/packages/python.scm b/gnu/packages/python.scm
index 3dd3862..ae14404 100644
--- a/gnu/packages/python.scm
+++ b/gnu/packages/python.scm
@@ -796,7 +796,7 @@ Python 3 support.")
 (define-public python-pycrypto
   (package
     (name "python-pycrypto")
-    (version "2.6.1")
+    (version "2.8.7")
     (source
      (origin
       (method url-fetch)
@@ -1565,7 +1565,7 @@ code introspection, and logging.")
 (define-public python-pytest
   (package
     (name "python-pytest")
-    (version "2.6.1")
+    (version "2.8.7")
     (source
      (origin
        (method url-fetch)
@@ -1574,7 +1574,7 @@ code introspection, and logging.")
              version ".tar.gz"))
        (sha256
         (base32
-         "0g2w4p0n42wvz8rq4k6gnzpkakgz3g8sfanxk8jrsra9675snkcr"))
+         "1bwb06g64x2gky8x5hcrfpg6r351xwvafimnhm5qxq7wajz8ck7w"))
        (modules '((guix build utils)))
        (snippet
         ;; One of the tests involves the /usr directory, so it fails.