Add a CLI command to list packages.

these two lines are not necessary. Merely accessing new attributes of a ``MagicMock`` instance causes it to create a new ``MagicMock`` instance, assign it to that attribute name, and return it.

```
>>> import mock
>>> m = mock.MagicMock()
>>> m.a
<MagicMock name='mock.a' id='140302825065488'>
>>> m.b
<MagicMock name='mock.b' id='140302825074320'>
>>> m.a.b.c.d
<MagicMock name='mock.a.b.c.d' id='140302825121424'>
```