You could always just download the source via github and place it under the lessons
git clone https://github.com/fastai/fastai_docs
git clone https://github.com/fastai/fastai
cd fastai_docs/dev_course/dl2
ln -s ../../../fastai/fastai .
so now when you load a notebook from fastai_docs/dev_course/dl2 it will use these local fastai modules since '' (nb dir) is always in sys.path. This way you don’t have any dependency conflicts to deal with since you’re not using any package manager here.
This is more or less what you suggested you did above, just easier since you don’t need to go and fish out specific files from the fastai modules.