Search layout file in nested modules

```php
'modules' => array(
  'admin' => array(
    'layout' => 'admin',
    'modules' => array(
      'user' => array(
    ),
  ),
)
```


```
app
  module1
    module2
```

For module inside a module layout was searched only in the current module (module2).

With this PR it's searched in the following sequence: `module2 → module1 → app` if module's `layout===null`.