Pushes autoenv code to its plugin.

Sources the activate.sh file or informs the user, that its not
available.
Overriding the cd function, as it was formerly done, is not needed
anymore, activate.sh does this by itself.

plugins/autoenv/autoenv.plugin.zsh

+# Activates autoenv or reports its failure
+if ! source $HOME/.autoenv/activate.sh 2>/dev/null; then
+  echo '-------- AUTOENV ---------'
+  echo 'Could not find ~/.autoenv/activate.sh.'
+  echo 'Please check if autoenv is correctly installed.'
+  echo 'In the meantime the autoenv plugin is DISABLED.'
+  echo '--------------------------'
+  return 1
+fi
+
 # The use_env call below is a reusable command to activate/create a new Python
 # virtualenv, requiring only a single declarative line of code in your .env files.
 # It only performs an action if the requested virtualenv is not the current one.
+
 use_env() {
     typeset venv
     venv="$1"