Correct `yii\base\Theme::pathMap` generation when its not provided

This allow not provide `pathMap` in 90% cases. So theme configuring become simplier, like:

```
'components' => [
    'view' => [
        'theme' => [
            'basePath' => '@app/themes/basic',
            'baseUrl' => '@web/themes/basic',
        ],
    ],
],
```