We hit a situation during a deployment where $sitedir/venv, which

was already a working site directory, was replaced with a symlink to

itself, erasing the entire environment in the process and requiring a

server rebuild.
This directory is too important to risk. We don't want to impact it if

it already exists, whether it's the site's virtualenv directory or not.
We now skip the copying/symlinking phase if this already exists.

Deployed a new copy of the site and verified it didn't result in the

venv directory being replaced.