--system-site-packages doesn't run inside snaps

[facundobatista]

This is because the intrinsic nature of isolation provided by snaps.

However, as fades is currently a "classic" snap, we can really access to external Python.

I did some exploration here, playing with getting dist/site packages of external Python and adding it to the running virtualenv inside fades (which theoretically would make it add those paths to venv's site.py, in effect providing access to "the system packages", but I couldn't really make it work... it looks I break the virtualenv containment when I do that...