Extensions example: ModuleNotFoundError: No module named 'mypackage'

I got ModuleNotFoundError when I was making an extension to notebook. This notebook is build by myslef, and installed from anaconda has this err too.

I just append my current path to sys.path in notebookapp.py solved my problem right now.

Version:
notebook 5.6.0
juypter 4.4.0
ipython 6.4.0
python 3.6.6
sierra 10.12.6

Start with the extension-loading path in notebookapp.py and reproduce the ModuleNotFoundError using the reported notebook 5.6.0, Jupyter 4.4.0, and Python 3.6.6 versions. Trace how the notebook process resolves the extension package; done means an extension can import its package without manually appending the current path to sys.path.

Written by the indexing model from the issue text.