Don't required BaseYii manual

Let Composer doing his job.

Because I need the BaseYii class before importing Yii, it cause redeclare class error.

If you insist on imports, for users which don't use Composer, please check if class exists before import file.

```php
if (class_exists('\yii\BaseYii', false)) {
    require(__DIR__ . '/BaseYii.php');
}
```