title: Transition from single to multi module project with python SIP
body: <p>I have trouble doing an import of foo_a in foo_b. Lets say the toplevel-package defined in both the .toml files will be called foo.sip.
  That means e.g. the pyproject.toml of foo_b looks like this:</p>
  <pre><code># Specify sip v6 as the build system for the package.
  [build-system]
  requires = [&quot;sip &gt;=6, &lt;7&quot;]
  build-backend = &quot;sipbuild.api&quot;
  
  # Specify the PEP 566 metadata for the project.
  [tool.sip.metadata]
  name = &quot;foo-foo_b&quot;
  requires-dist = &quot;foo-foo_a&quot;
  
  # Configure the building of the project bindings.
  [tool.sip.bindings.foo_b]
  &lt;dependencies ...&gt;
  
  [tool.sip.project]
  sip-module = &quot;foo.sip&quot;
  </code></pre>
  <p>I have a the following project structure:</p>
  <ul>
  <li><p>dir_a</p>
  <ul>
  <li>foo_b.cpp</li>
  <li>foo_b.h</li>
  <li>pyproject.toml</li>
  <li>foo_b.sip   &lt;-- here I try to %import foo_a.sip</li>
  <li>test_b.py</li>
  </ul>
  </li>
  <li><p>dir_b</p>
  <ul>
  <li>foo_a.cpp</li>
  <li>foo_a.h</li>
  <li>pyproject.toml</li>
  <li>foo_a.sip</li>
  <li>test_a.py</li>
  </ul>
  </li>
  </ul>
  <p>If I remove the sip-module entry under [tool.sip.project]</p>
  <p><code>sip-module = foo.sip</code></p>
  <p>from the .toml file of foo_a, the import of foo_a</p>
  <p><code>from foo import foo_a</code></p>
  <p>in test_a.py works fine. But if I try to build a package project with this entry, the following error occurs:</p>
  <p><code>ModuleNotFoundError: No module named 'foo.sip'</code></p>
  <p>The installation on the system is done with <code>sip-install</code>, first in dir_a and then in dir_b.</p>
  <p><a href="https://www.riverbankcomputing.com/static/Docs/sip/examples.html#package-projects" rel="nofollow noreferrer">I tried to structure my package project like it is in this example more or less described</a></p>
  <p>But what am I missing?</p>