New functions in separate directory — better, portable way

Question

I’d say that the thing you’re looking for is having functions.php as a plugin. This makes it portable, regardless of the theme you are using. The most simple way is create a new file (something like my-woocommerce-functions.php) and start with the following:

<?php
/**
 * Plugin Name: My Custom WooCommerce Functions
 * Plugin URI: http://example.com
 * Description: My Custom WooCommerce Functions
 * Author: Your Name
 * Author URI: http://example.com
 * Version: 1.0.0
 */

/* Place custom code below this line. */
require get_template_directory() . '/woocommerce/storefunctions/woocommerce-overrides.php';
?>

Now you have a plugin which you can activate and deactivate from dashboard. You would have to take care about file paths only (if not using get_template_directory() function).