title: Collapse multiple submodules to one Cython extension
body: <p>This setup.py:</p>
  
  <pre class="lang-py prettyprint-override"><code>from distutils.core import setup
  from distutils.extension import Extension
  from Cython.Build import cythonize
  
  extensions = (
      Extension('myext', ['myext/__init__.py',
                          'myext/algorithms/__init__.py',
                          'myext/algorithms/dumb.py',
                          'myext/algorithms/combine.py'])
  )
  setup(
      name='myext',
      ext_modules=cythonize(extensions)
  )
  </code></pre>
  
  <p>Doesn't have the intended effect. I want it to produce a single <code>myext.so</code>, which it does; but when I invoke it via</p>
  
  <pre class="lang-none prettyprint-override"><code>python -m myext.so
  </code></pre>
  
  <p>I get:</p>
  
  <pre class="lang-none prettyprint-override"><code>ValueError: Attempted relative import in non-package
  </code></pre>
  
  <p>due to the fact that <code>myext</code> attempts to refer to <code>.algorithms</code>.</p>
  
  <p>Any idea how to get this working?</p>