Can’t move custom page builder widgets from theme to plugin

I’ve taken some custom SiteOrigin Page Builder widgets out of a theme and placed them in a new custom plugin. We are moving to a different theme.

In the main plugin file, I have:

require plugin_dir_url( __FILE__ ) . 'hdr003.php';
require plugin_dir_url( __FILE__ ) . 'hme006.php';
require plugin_dir_url( __FILE__ ) . 'hme007.php';

function purity_register_widgets(){
    register_widget('ICD_HDR003_Widget');
    register_widget('ICD_HME006_Widget');
    register_widget('ICD_HME007_Widget');
}
add_action( 'widgets_init', 'purity_register_widgets');

In, for example, hdr003.php, I have:

class ICD_HDR003_Widget extends WP_Widget {

However, I receive:

Fatal error: Class ‘WP_Widget’ not found

Hence, I’m trying to include in hdr003.php:

require_once(get_home_path(). 'wp-includes/class-wp-widget.php');

but this produces an error:

Fatal error: Uncaught Error: Call to undefined function get_home_path()

The function is a valid function.

If I replace that line with:

require_once(ABSPATH . 'wp-includes/class-wp-widget.php');

I receive:

include_once(ABSPATH/wp-includes/class-wp-widget.php): failed to open stream: No such file or directory

The file /wp-includes/class-wp-widget.php exists.

I’ve replaced /wp-admin & /wp-includes with newly downloaded copies.

Help appreciated.