Modify installation extension dependencies (#518)
Perform a full extension installation process instead of just installing
dependencies
Some extensions do not include `requirements.txt` but install
dependencies in `install.py`, and all extensions include `install.py`,
so it is safe to use it for extended dependency installation
This is because the extension development of AUTOMATIC1111's webui does
not require the existence of `requirements.txt` but uses `install.py` to
initialize the extension
https://github.com/AUTOMATIC1111/stable-diffusion-webui/wiki/Developing-extensions#installpy