How to include files relative to a file’s directory

Plugin within a theme – sounds like something new. You might not find function in this context. Let’s create ours:

<?php
/**
 * Get My Plugin URL.
 * @return string.
 */
function wpse295740_get_plugin_url() {
    // http://example.com/wp-content/themes/my-theme/inc/my-plugin
    return get_template_directory_uri() .'/inc/my-plugin';
}

/**
 * Get My Plugin Path.
 * @return string.
 */
function wpse295740_get_plugin_path() {
    // eg. /home/user/var/www/wordpress/wp-content/themes/my-theme/inc/my-plugin
    return get_template_directory() .'/inc/my-plugin';
}

You can make it more complex using the __FILE__ magic constant etc. – see the original plugin_url() source. And plugin_dir_path() too. And trailingslashit() and untrailingslashit() are there too for ease.