Dependencies cleanup

Created by: ChrisSchinnerl

This PR is supposed to clean up the dependencies interface of Sia. Currently we have a dependencies.go file for every (sub)module that uses the dependencies interface. That means we have the same interface and same production implementation in multiple files. In this PR all of the dependencies and productionDependencies interfaces/implementations are merged into one single top layer dependencies.go file. That way we have a single point of entry for people trying to understand the dependencies system and can also provide a nice description in for GoDocs. Custom dependencies should be created by embedding the modules.ProductionDependencies and overwriting the required method.