title: Django migrations reference a deleted module
body: <p>I have a <code>Model</code> named <code>FooModel</code> defined in a <code>my_app/models/foo.py</code>.</p>
  
  <p>After deleting <code>foo.py</code>, running Django (1.7) migrations raises an error since the old migration files import <code>foo.py</code> (<code>import myapp.models.foo.FooModel</code>).</p>
  
  <p>How should I resolve this?</p>
  
  <p>This happens when the model has an <code>ImageField</code> with an <code>upload_to</code> parameter.</p>