The type or namespace name 'OmnichannelProviders' could not be found

Hi there! We are developing an application and want to add a custom channel provider. We follow this guide, and stuck on 4. Implement the conversion of an incoming message step. 

After adding completely the same converted class and publishing it, we receive 

The type or namespace name 'OmnichannelProviders' could not be found (are you missing a using directive or an assembly reference?) compilation result. 

As the guide suggests, this class should be added to Custom package and it is working ok in this case, however we need to add it to our application's package so it can be exported and installed. Is it possible?