composer: autoload cache functions

Fixes #9236

plugins/composer/composer.plugin.zsh

@@ -51,6 +51,8 @@ alias cget='curl -s https://getcomposer.org/installer | php'
 
 # Add Composer's global binaries to PATH, using Composer if available.
 if (( $+commands[composer] )); then
+    autoload -Uz _store_cache _retrieve_cache
+
     _retrieve_cache composer
 
     if [[ -z $__composer_bin_dir ]]; then