Use the Prefix path for the environment when autofinding projects

[karthiknadig]

          Actually, we need to use the Prefix path for the environment. This is not always good enough. I think for now `.venv` is ok, but really we should use the sysPrefix path for environment for the workspace and ignore all `pyproject.toml` or `setup.py` from there.

Originally posted by @karthiknadig in #405 (comment)