python.pkgs.virtualenv: move to separate expression

pkgs/development/python-modules/virtualenv/default.nix

···
+{ buildPythonPackage
+, fetchPypi
+, lib
+, recursivePthLoader
+}:
+
+buildPythonPackage rec {
+  pname = "virtualenv";
+  version = "15.0.3";
+  name = "${pname}-${version}";
+
+  src = fetchPypi {
+    inherit pname version;
+    sha256 = "6d9c760d3fc5fa0894b0f99b9de82a4647e1164f0b700a7f99055034bf548b1d";
+  };
+
+  # Doubt this is needed - FRidh 2017-07-07
+  pythonPath = [ recursivePthLoader ];
+
+  patches = [ ./virtualenv-change-prefix.patch ];
+
+  # Tarball doesn't contain tests
+  doCheck = false;
+
+  meta = {
+    description = "A tool to create isolated Python environments";
+    homepage = http://www.virtualenv.org;
+    license = lib.licenses.mit;
+    maintainers = with lib.maintainers; [ goibhniu ];
+  };
+}

pkgs/top-level/python-packages.nix

···
     };
   };
 
-  virtualenv = buildPythonPackage rec {
-    name = "virtualenv-15.0.3";
-
-    src = pkgs.fetchurl {
-      url = "mirror://pypi/v/virtualenv/${name}.tar.gz";
-      sha256 = "6d9c760d3fc5fa0894b0f99b9de82a4647e1164f0b700a7f99055034bf548b1d";
-    };
-
-    pythonPath = [ self.recursivePthLoader ];
-
-    patches = [ ../development/python-modules/virtualenv-change-prefix.patch ];
-
-    propagatedBuildInputs = with self; [ ];
-
-    # Tarball doesn't contain tests
-    doCheck = false;
-
-    meta = {
-      description = "A tool to create isolated Python environments";
-      homepage = http://www.virtualenv.org;
-      license = licenses.mit;
-      maintainers = with maintainers; [ goibhniu ];
-    };
-  };
+  virtualenv = callPackage ../development/python-modules/virtualenv { };
 
   virtualenv-clone = buildPythonPackage rec {
     name = "virtualenv-clone-0.2.5";