refactor(plugins): use cached background completion generation

plugins/kn/kn.plugin.zsh

@@ -1,8 +1,17 @@
-# Autocompletion for kn, the command line interface for knative
+# Autocompletion for kn, the command line interface for Knative.
 #
 # Author: https://github.com/btannous
 
-if [ $commands[kn] ]; then
-  source <(kn completion zsh)
-  compdef _kn kn
+if (( ! $+commands[kn] )); then
+  return
 fi
+
+# If the completion file doesn't exist yet, we need to autoload it and
+# bind it to `kn`. Otherwise, compinit will have already done that.
+if [[ ! -f "$ZSH_CACHE_DIR/completions/_kn" ]]; then
+  typeset -g -A _comps
+  autoload -Uz _kn
+  _comps[kn]=_kn
+fi
+
+kn completion zsh >| "$ZSH_CACHE_DIR/completions/_kn" &|