Move Feed targets logic into the Arcade SDK

https://github.com/dotnet/arcade/pull/1908#issuecomment-459805002

> if you move all targets logic to Publish.proj and only expose tasks from the package you'll have to define and maintain only the tasks API. It this thus going to be simpler.
I agree that the current state is not desirable. I'd suggest that instead of importing the targets file defined in package here we actually remove it from the package and move all the logic here.