[python] Detect availability of virtualenv plugins as well as pyenv

Just having 'pyenv' available doesn't imply availability of virtualenv pyenv
plugins. Check for availability of virtualenv plugins as well before attempting
to use pyenv wrapper for virtualenv.

modules/python/init.zsh

@@ -92,16 +92,16 @@ if (( $+VIRTUALENVWRAPPER_VIRTUALENV || $+commands[virtualenv] )) && \
   # Disable the virtualenv prompt.
   VIRTUAL_ENV_DISABLE_PROMPT=1
 
-  if (( $+commands[pyenv] )); then
-    if (( $+commands[pyenv-virtualenv-init] )); then
+  # Enable 'virtualenv' with 'pyenv'.
+  if (( $+commands[pyenv] && $+commands[pyenv-virtualenv-init] )); then
     eval "$(pyenv virtualenv-init -)"
-    fi
+    # Optionall activate 'virtualenvwrapper' with 'pyenv' is available.
     if (( $#commands[(i)pyenv-virtualenvwrapper(_lazy|)] )); then
       pyenv "${${(@O)commands[(I)pyenv-virtualenvwrapper(_lazy|)]}[1]#pyenv-}"
     fi
   else
-    # Try 'virtualenvwrapper' without 'pyenv' wrapper in '$path' and other
-    # known locations on a Debian based system.
+    # Fallback to 'virtualenvwrapper' without 'pyenv' wrapper in '$path'
+    # and other known locations on a Debian based system.
     virtenv_sources=(
       ${(@Ov)commands[(I)virtualenvwrapper(_lazy|).sh]}
       /usr/share/virtualenvwrapper/virtualenvwrapper(_lazy|).sh(OnN)