Thank you!! 
@regis This really needs to be documented somewhere.
The problem is also documented in post Tutor dev - fail to install app plugin.
It wasn’t enough to just add the .egg-info folder. I had to generate the folder contents, as well.
# install and create the virtual environment
sudo apt install python3-venv
python3.8 -m venv ~/venv3.8
# enter the virual env
source ~/venv3.8/bin/activate
# cd to xblock dev directory
# cd $(tutor config printroot)/env/build/openedx/requirements/xblock_dev
# Generate the .egg-info folder
# https://stackoverflow.com/questions/12901776/python-setup-py-develop-is-it-possible-to-create-egg-info-folder-not-in-so
sudo python3 setup.py develop
# leave the virtual env
deactivate