Create Python venv instead of manual site-packages

[chhtz]

Work in progress

[chhtz]

This is in a mostly working state, some possible improvements:

• Code should be moved to autoproj directly, all functions in this package set should call the corresponding autoproj function and get marked deprecated
• Code generally could be tested more, especially what happens when deactivating/re-activating
  • Make sure no illogical options are possible (activate venv without activating python)
  • Also, what happens if venv is (partially) deleted manually, does it recover by an autoproj osdep?
• Make sure no downloads/installation happens, except in the osdeps-phase
• Location of venv could be made configurable (in case one wants to share it between workspaces -- only works if installed versions are identical)