Only source varstash lib if it's being used

I have noticed that `autostash` called an (accidentally)
defined/overwritten `stash` function.

While it's not possible to put this into a local scope, this will at
least make it less polluting by default (when autostashing is not used).

Additionally we could source it always (not just once), but then it
would only re-overwrite the function.

A better fix would be to prefix the functions explicitly, e.g.
`autoenv_autostash` etc.

autoenv.zsh

@@ -260,8 +260,12 @@ _autoenv_source() {
   local autoenv_to_dir=$PWD
 
   # Source varstash library once.
+  # XXX: pollutes environment with e.g. `stash`, and `autostash` will cause
+  # an overwritten `stash` function to be called!
   if [[ -z "$functions[(I)autostash]" ]]; then
+    if \grep -qE '\b(autostash|autounstash|stash)\b' $autoenv_env_file; then
       source ${${funcsourcetrace[1]%:*}:h}/lib/varstash
+    fi
     # NOTE: Varstash uses $PWD as default for varstash_dir, we might set it to
     # ${autoenv_env_file:h}.
   fi
@@ -331,7 +335,9 @@ _autoenv_chpwd_handler() {
         fi
 
         # Unstash any autostashed stuff.
+        if [[ -n "$functions[(I)autostash]" ]]; then
           varstash_dir=$prev_dir autounstash
+        fi
 
         _autoenv_stack_entered_remove $prev_file
       fi