title: Python homemade package import error after pip install
body: <p>If I make a very simple package like this (with empty <code>__init__.py</code> files):</p>
  
  <pre><code>package_name/
      package_name/
           sub_module/
                __init__.py
                file.py  
           __init__.py
      setup.py
  </code></pre>
  
  <p>When I start a Python shell at the root of this project, and do:</p>
  
  <pre><code>import package_name.sub_module.file
  </code></pre>
  
  <p>everything goes well and I can use the functions in <code>file.py</code>. But if I send this project on a GitLab, and pip install it in another local project and import it the same way, I get the following error: </p>
  
  <pre><code>ModuleNotFoundError: No module named 'package_name.sub_module'
  </code></pre>
  
  <p>I tried to modify the <code>__init__.py</code> files in many ways but I can't find something working. I don't understand why this happen. </p>
  
  <p><strong>EDIT:</strong> Here is the content of the <code>setup.py</code> file:</p>
  
  <pre><code>from setuptools import setup
  
  setup(name='package_name',
        version='0.1',
        description='My package',
        url='https://gitlab.myserver.com/package_name',
        author='Me',
        author_email='me@myserver.com',
        license='MIT',
        packages=['package_name'],
        install_requires=[
            'another_package_1',
            'another_package_2'
        ],
        zip_safe=False)
  </code></pre>
  
  <p>To install the package with pip, I use the command:</p>
  
  <pre><code>pip install git+https://gitlab.myserver.com/package_name.git
  </code></pre>