This was introduced when merging the rbenv upstream change
(https://github.com/sstephenson/rbenv/commit/6bb7f07d2d).

This adds the missing parts from the upstream rbenv merge:
f4652fbbf0

- e4cbf04592
- 3ee395f9b5

This greatly improves the performance of `pyenv virtualenvwrapper_lazy`,
which happens to call pyenv-which a lot.

For profiling I've initially used:

    % zmodload zsh/zprof
    % eval "$(pyenv init -)"
    % pyenv virtualenvwrapper_lazy

    Before:
    % zprof|grep -E '(pyenv|virtualenv)'
     1)    1         754,07   754,07   58,95%    751,50   751,50   58,75%  pyenv
    21)    1           2,57     2,57    0,20%      2,57     2,57    0,20%  virtualenvwrapper_setup_lazy_loader
     1)    1         754,07   754,07   58,95%    751,50   751,50   58,75%  pyenv
           1/1         2,57     2,57    0,20%      2,57     2,57             virtualenvwrapper_setup_lazy_loader [21]
           1/1         2,57     2,57    0,20%      2,57     2,57             pyenv [1]
    21)    1           2,57     2,57    0,20%      2,57     2,57    0,20%  virtualenvwrapper_setup_lazy_loader

    After:
    % zprof|grep -E '(pyenv|virtualenv)'
     1)    1         383,30   383,30   27,97%    380,88   380,88   27,79%  pyenv
    31)    1           2,42     2,42    0,18%      2,42     2,42    0,18%  virtualenvwrapper_setup_lazy_loader
     1)    1         383,30   383,30   27,97%    380,88   380,88   27,79%  pyenv
           1/1         2,42     2,42    0,18%      2,42     2,42             virtualenvwrapper_setup_lazy_loader [31]
           1/1         2,42     2,42    0,18%      2,42     2,42             pyenv [1]
    31)    1           2,42     2,42    0,18%      2,42     2,42    0,18%  virtualenvwrapper_setup_lazy_loader

Fixes https://github.com/yyuu/pyenv-virtualenvwrapper/issues/13

users can use multiple python versions at once.

modified rbenv source for Python and renamed to pyenv.