title: importlib not working for files in subdirectory
body: <p>I have a directory structure like this :</p>
  
  <pre><code>__init__.py
  test.py
  dir1/
      __init__.py
      dir2
          __init__.py
          myfile.py
  </code></pre>
  
  <p>There might be more py files inside dir2. So I want to import them at run time and load all classes defined inside these files.</p>
  
  <p>test.py:</p>
  
  <pre><code>import inspect
  import importlib
  
  module = importlib.import_module('dir1/dir2/myfile.py')
  for name, obj in inspect.getmembers(module):
      if inspect.isclass(module):
          print(obj.id)   # id is defined in all the classes
  </code></pre>
  
  <p>This gives me error while doing import_module :</p>
  
  <blockquote>
    <p>ModuleNotFoundError: No module named 'dir1/dir2/myfile'</p>
  </blockquote>
  
  <p>I've tried to append dir1/dir2 path to sys.path and then import myfile.py, which doesn't work either. Also the similar code works is myfile.py is placed at same level as test.py.</p>
  
  <p>Python version : 3.7</p>