title: Can't find submodule within package
body: <p>I feel a bit dumb asking this since there are a lot of similar questions, but I've honestly searched around a lot and could not find a solution for this. Here goes:</p>
  
  <p>I have a Python package (on <a href="https://test.pypi.org/project/rens-paillier/" rel="nofollow noreferrer">TestPyPi</a>, <a href="https://gitlab.com/RensOliemans/paillier" rel="nofollow noreferrer">here</a> is the source code, note that it uses and needs python3.8) with the following structure:</p>
  
  <pre><code>paillier/
      setup.py
      test/
      paillier/
          __init__.py
          keygen.py
          util/
              __init__.py
              math_shortcuts.py
  </code></pre>
  
  <p>My use case is: in <code>keygen.py</code>, I want to use <code>util/math_shortcuts.py</code>.<br>
  So, in <code>keygen.py</code>, I have the following import:
  <code>from paillier.util.math_shortcuts import generate_coprime, lcm, get_mu</code>.<br>
  However, when I try to use my package (by doing <code>from paillier.keygen import generate_keys</code>), I am greeted with the Error <code>ModuleNotFoundError: No module named 'paillier.util'</code></p>
  
  <p>This ModuleNotFoundError is always present when I install it using <code>pip</code> from <code>TestPyPi</code>, but it doesn't happen when I build the package locally: when I run <code>pip install -e .</code> in the <code>paillier/</code> directory (where <code>setup.py</code> lives), I <em>can</em> run <code>from paillier.keygen import generate_keys</code>, even when my working directory is somewhere else.</p>
  
  <p>I've tried to do <code>from .util.math_shortcuts ...</code>, or <code>from util.math_shortcuts ...</code>, or <code>from paillier.paillier.util.math_shortcuts ...</code>, but all to no avail.</p>
  
  <hr>
  
  <p>In short, when doing <code>pip install --index-url &lt;TestPyPi&gt; rens-paillier</code> my files can't seem to find the submodules.<br>
  However, when doing <code>pip install -e .</code> in the outer <code>paillier/</code> dir, it seems to work.</p>