virtualenv things are poorly named given existence py3 stdlib venv

[kyleam]

I used the short "venv" for a lot of virtual environment things (venv.py, VenvTracer, etc). But that's confusing because the py3 stdlib has its own venv module. We should rename the existing virtualenv things and perhaps eventually add support for the stdlib venv (Unsurprisingly, given my naming choices, I have no experience with venv.)