title: Module can't be found when called from outside
body: <p>I have a multiple sub-modules python project. For example, "myproject" which has two submodules namely "submodule1" and "submodule2".</p>
  
  <p>Project structure</p>
  
  <pre><code>Example1/
  |-- submodule1/
  |   |-- __init__.py
  |   |-- hello.py
  ...
  |-- submodule2/
  |   |-- __init__.py
  ...
  </code></pre>
  
  <p>And I have a <code>hello.py</code> in <code>submodule1</code> which has the following content:</p>
  
  <pre><code>import datetime
  
  def greeting():
      print("hello world! - " + datetime.datetime.now().strftime("%Y-%m-%d %H:%M:%S"))
  </code></pre>
  
  <p>The content for <code>submodule1</code>'s <code>__init__.py</code> is:</p>
  
  <pre><code>def main():
      import hello
      hello.greeting()
  
  if __name__ == '__main__':
      main()
  </code></pre>
  
  <p>I am able to import <code>hello.py</code> and call <code>greeting()</code> function successfully here.</p>
  
  <p>Now, I installed <code>submodule1</code> as a package to python, and then created <code>submodule2</code>, and tried to call <code>submodule1</code> from <code>submodule2</code> by putting following code in <code>__init__.py</code> from <code>submodule2</code>:</p>
  
  <pre><code>import submodule1
  
  def main():
      submodule1.main()
  
  if __name__ == '__main__':
      main()
  </code></pre>
  
  <p>It fails with this message: </p>
  
  <pre><code>....in main submodule1.main()
      ModuleNotFoundError: No module named 'hello'
  </code></pre>
  
  <p>I think <code>hello</code> shouldn't be exposed to outside, how am I able to let my <code>submodule1</code> works? I'm using python3</p>
  
  <p>========</p>
  
  <p>setup.py is:</p>
  
  <pre><code>from setuptools import setup, find_packages
  
  with open('requirements.txt') as f:
      requirements = f.read().splitlines()
  
  __version__ = "0.0.2"
  
  setup(
      version=__version__
      name = "mymodules",
      packages = find_packages(exclude=['tests', '*.tests', '*.tests.*']),
      install_requires=requirements,
  )
  </code></pre>