import: hackage: Do not repeat inputs in native-inputs.

* guix/import/hackage.scm (hackage-module->sexp): Do not repeat inputs again
in native-inputs. native-inputs should only contain packages that are not
already listed in inputs.

guix/import/hackage.scm

@@ -215,6 +215,8 @@ representation of a Cabal file as produced by 'read-cabal'."
      cabal))
 
   (define hackage-native-dependencies
+    (lset-difference
+     equal?
      ((compose (cut filter-dependencies <>
                     (cabal-package-name cabal))
                ;; FIXME: Check include-test-dependencies?
@@ -223,7 +225,8 @@ representation of a Cabal file as produced by 'read-cabal'."
                              (cabal-test-dependencies->names cabal)
                              '())
                          (cabal-custom-setup-dependencies->names cabal))))
-     cabal))
+      cabal)
+     hackage-dependencies))
 
   (define dependencies
     (map (lambda (name)