espidf installer: Check that python has venv package

[N3xed]

In the esp-idf installer when it tries to create a python virtual env with

python esp-idf-v4.3.1/tools/idf_tools.py --idf-path esp-idf-v4.3.1 --quiet --non-interactive install-python-env

if the venv package is not installed the idf_tools.py script fails.

Note that the error message is also misleading:

ERROR: This script was called from a virtual environment, can not create a virtual environment again

Desired state:
Check for this case and output a specific error (with instructions) early.