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
1. It results in much smaller download and takes less disk space (25MB instead of 180MB for yii-basic + dependencies).
2. It requires less utilities to be installed (no VCS binaries required).
3. It takes less time to install dependencies.