Avoid recreation of virtualenv due to symlinks

[ssbarnea]

If people called tox alternating between python3 -m tox and tox,
they would endup triggering venv recreation, even if there was only
one python. We should always resolve symlinks to avoid such false
env invalidations.

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[ssbarnea]

@gaborbernat I would have written a test but I am not sure how to test for that. Maybe you can indicate me an existing test that loads the info and we can check that executable is already resolved inside the file?