call hasMethod() instead of using method_exists

allow extending only one class to add scopes, fixes #2146

TODO:

- [ ] adjust guide docs
- [ ] adjust README files of extensions
- [ ] finish work and fix test breaks

Fixes #2490: `yii\db\Query::count()` and other query scalar methods now properly handle queries with GROUP BY clause

#2436 override the function `getAttributeLabel` in `BaseActiveRecord` to receive label of the attribute, which looks like `relatedMode.attribute`, from the related model

#2436 override the function `getAttributeLabel` in `BaseActiveRecord` to receive label of the attribute, which looks like `relatedMode.attribute`, from the related model

fixes #2370