code: def _load_mapping() -> Optional[Dict[str, str]]:
          
          if not pipreqs:
              return None
          path = os.path.dirname(inspect.getfile(pipreqs))
          path = os.path.join(path, 'mapping')
          with open(path) as f:
              # pypi_name: import_name
              mappings = {}  # type: Dict[str, str]
              for line in f:
                  import_name, _, pypi_name = line.strip().partition(":")
                  mappings[pypi_name] = import_name
              return mappings
docs: Return list of mappings `package_name -> module_name`
  
          Example:
              django-haystack -> haystack