spack setup-env.sh: make zsh loading async compatible, and ~10x faster (in some cases) (#26120)

Currently spack is a bit of a bad actor as a zsh plugin, and it was my
fault.  The autoload and compinit should really be handled by the user,
as was made abundantly clear when I found spack was doing completion
initialization for *all* of my plugins due to a deferred setup that was
getting messed up by it.

Making this conditional took spack load time from 1.5 seconds (with
module loading disabled) to 0.029 seconds. I can actually afford to load
spack by default with this change in.

Hopefully someday we'll do proper zsh completion support, but for now
this helps a lot.

* use zsh hist expansion in place of dirname
* only run (bash)compinit if compdef/complete missing
* add zsh compiled files to .gitignore
* move changes to .in file, because spack

share/spack/bash/spack-completion.in

@@ -39,10 +39,14 @@
 
 if test -n "${ZSH_VERSION:-}" ; then
   if [[ "$(emulate)" = zsh ]] ; then
-    # ensure base completion support is enabled, ignore insecure directories
-    autoload -U +X compinit && compinit -i
-    # ensure bash compatible completion support is enabled
-    autoload -U +X bashcompinit && bashcompinit
+    if ! typeset -f compdef >& /dev/null ; then
+        # ensure base completion support is enabled, ignore insecure directories
+        autoload -U +X compinit && compinit -i
+    fi
+    if ! typeset -f complete >& /dev/null ; then
+        # ensure bash compatible completion support is enabled
+        autoload -U +X bashcompinit && bashcompinit
+    fi
     emulate sh -c "source '$0:A'"
     return # stop interpreting file
   fi