remove sanity check for existence of include which isn't always there

Assuming include is always there, it's a reasonable check to do in tests,
but it isn't and I don't think it matters enough to fail the test, so why
is it there?

(And here's another reminder that it's really not possible to determine if
a dir is a virtualenv without just using it)
pull version handling out into own function

no real reason, just tidying - will do --list the same way probably
draft of --list option

With optional prefix arg, because nothing else sensibly comes after
--list anyway, and the list might be very long.

I don't verify that anything 'really is' a virtualenv beyond isdir,
that is a slippery slope fraught with edge cases, and IMO people shouldn't
put other junk dirs in WORKON_HOME/ve_base anyway
simple functional test on output of --list