Avoid duplicate copy of labextension assets

[bollwyvl]

Elevator Pitch

Remove one of the copies of the labextension assets from the as-distributed wheels.

The current setup ships two copies of the labextension(s) in the wheel:

• once in data-files
• once in src/labextension
  • this is basically never needs to be used, other than during an interactive jupyter labextension develop

Motivation

This slows down the install by being larger on disk (.whl doesn't actually compress very much), and takes more space on disk.

Design Ideas

• in paths.py and __init__.py
  • during develop, we should be able to detect whether assets exist in the source layout
    • e.g. python_packages/... and core
  • otherwise, always source the assets from the standard data locations
    • whichever is found first (by user preference) in e.g.
    • [jupyter_core.get_data_dir(), *jupyter_paths.jupyter_path()]