Standardise on using virtualenv package (#2666)

Until now poetry used the built-in venv module available in Python 3.
This has presented a few concerns. Chief of which has been
inconsistent environment setup. This includes, but is not limited to,
pip version used by the `ensurepip` module which is in turn used by the
built-in `venv.EnvBuilder`. Additionally, the `virtualenv` package
allows for faster environment creation. This can also allow us to, going
forward, drop ad-hoc code required to inspect and manage environments.