Fix for cases in the virtualenv plugin where pyenv is present but pyenv virtualenvwrapper isn't

[LoKolbasz]

Description

Check for the whole pyenv virtualenvwrapper command when loading virtualenv, instead of checking for pyenv only

Motivation and Context

This is useful if the system has pyenv installed but not pyenv virtulenvwrapper. The previous version just printed out

pyenv: no such command `sh-virtualenvwrapper'

which is just the pyenv stderr output.

How Has This Been Tested?

Tested both with and pyenv virtualenvwrapper present. In both cases virtualenvwrapper.sh was also present. In poth cases ´ _virtualenvwrapper_version´ returned with the verion value. I renamed the plugins in enabled, to ensure that pyenv gets loaded first during debugging. I don't remember the original loading order, but pyenv must be loaded first so that the plugin would work. Otherwise pyenv prints the following:

Failed to initialize virtualenvwrapper.

Perhaps pyenv-virtualenvwrapper has not been loaded into your shell properly.
Please restart current shell and try again.

Screenshots (if appropriate):

Types of changes

• [ x] Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)

Checklist:

• [ x] My code follows the code style of this project.
• [ x] If my change requires a change to the documentation, I have updated the documentation accordingly.
• [ x] I have read the CONTRIBUTING document.
• [ x] If I have added a new file, I also added it to clean_files.txt and formatted it using lint_clean_files.sh.
• I have added tests to cover my changes, and all the new and existing tests pass.

[seefood]

seems legit.