refactor: standardize command naming

This standardization makes it somewhat easier to read stacktraces as the command names are all uniform, so it will be slightly easier to scan trace output.

plugins/plugin/internal-functions

@@ -3,7 +3,7 @@ set -eo pipefail
 [[ $DOKKU_TRACE ]] && set -x
 source "$PLUGIN_CORE_AVAILABLE_PATH/common/functions"
 
-plugin_list_cmd() {
+cmd-plugin-list() {
   declare desc="lists all plugins"
   local cmd="plugin"