Reduce scope of the package registry as library

[jsoriano]

Sub-packages in the package registry are exposed also as modules. When making refactors in the registry this forces us to maintain backwards compatibility, or risk breaking dependent projects, while the package registry is not generally intended to be used as library.

Move internal functionality to internal modules, and clarify what functionality can be used externally. For example it may make sense to keep public the structs that are mapped to JSON API objects. But almost any other functionality should be probably internal.

This change will be breaking for projects using the registry as library, and require to bump to a v2 version.