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Took me a while to figure this one out, and I have a default installation of virtualenvwrapper -- this is a soft fix, just put an error message. But perhaps the fix should be to use the default value `~/.virtualenvs`.

Fix robbyrussell#2355.

* removes cd override by using chpwd_functions
* removes subshell call to which by using $+commands array and
  c param expansion to find in PATH
* zsh love!

Avoid infinite `cd` loops under certain conditions. Try getting `.venv` from the current directory (not necessarily always using git)

Using lazy loading for virtualenvwrapper gives a mariginal speed
improvement and doesn't stop workon_cd from working. It has the
undesired effect of forcing you to call certain virtualenv commands
twice before they work (only once per shell instantiation).

When you install python via brew on OSX, pip is installed into /usr/local/share/python/ and all packages installed with this pip are installed here.