title: mypy Cannot find implementation or library stub for module
body: <p>I have:</p>
  <pre><code>foo/
  ├── __init__.py
  ├── bar.py
  └── baz
      ├── __init__.py
      └── alice.py
  </code></pre>
  <p>In <code>bar.py</code>, I import Alice, which is an empty class with nothing in it but the <code>name</code> attribute set to <code>&quot;Alice&quot;</code>.</p>
  <pre><code>from baz.alice import Alice
  
  a = Alice()
  print(a.name)
  </code></pre>
  <p>This runs properly:</p>
  <pre><code>$ python foo/bar.py
  Alice
  </code></pre>
  <p>But mypy complains:</p>
  <pre><code>$ mypy --version
  mypy 0.910
  $ mypy --strict .
  foo/bar.py:1: error: Cannot find implementation or library stub for module named &quot;baz.alice&quot;
  foo/bar.py:1: note: See https://mypy.readthedocs.io/en/stable/running_mypy.html#missing-imports
  Found 1 error in 1 file (checked 6 source files)
  </code></pre>
  <p>Why is mypy complaining?</p>