Comment # 2 on bug 1228165 from Matej Cepl

We can introduce a new option to the Python interpreter (let’s call it
tentatively -t as a next letter to -s; the other option is -ss), which would
ignore not only Python modules in ~/.local/lib/python*/site-packages, but also
/usr/local/lib*/python*/site-packages.

Then we can just not use EXTERNALLY_MANAGED at all.