Autoloader also looking for theme's class file.

Travelux Core

<?php
/**
 * Plugin Name:       Travelux Core
 * Plugin URI:        http://ithemeslab.com/
 * Description:       Extends the functionality of Travelux Theme.
 * Version:           1.0.0
 * Author:            iThemesLab
 * Author URI:        https://ithemeslab.com/
 * Text Domain:       travelux-core
 * Domain Path:       /languages
 */
?>
<?php
namespace TraveluxCore;

if ( ! defined( 'ABSPATH' ) ) {
    exit; // Exit if accessed directly.
}

final class Travelux_Core {

    private static $_instance = null;

    public static function instance() {

        if ( is_null( self::$_instance ) ) {
            self::$_instance = new self();
        }
        return self::$_instance;

    }

    public function __construct() {
        add_action( 'init', [ $this, 'i18n' ] );
        add_action( 'plugins_loaded', [ $this, 'init' ] );
		add_action( 'admin_enqueue_scripts', [ $this, 'admin_assets' ] );
    	$this->run();
	    $this->travelux_class_instantiate();
    }

    public function i18n() {
        load_plugin_textdomain( 'travelux-core', false, plugin_dir_url( __FILE__ ) . '/languages' );
    }

    public function autoload( $classname ){
    	$dirpath = plugin_dir_path( __FILE__ ) .'inc/class/';
	    $filename = $classname . '.php';
    	require_once ( $dirpath . $filename );
    }

    public function run(){
    	spl_autoload_register( [ $this, 'autoload' ], true );
    }

    public function travelux_class_instantiate(){
    	$tl_tour_post_type = new \TraveluxCore\Travelux_Tour_Post_Type();
    }
}

Travelux_Core::instance();