Refactor repository api

Currently, the repository access managed via the repository admin api and has several specifics in it that are highly dependent on the repository implementation.
At the time there is no way to provide another repository type without changing/duplicating code on a lot of different places.
Another issue, is that many modules are dependent on maven libraries.

Goal is to provide a central repository registry, that keeps track of repositories and delegates provider specific tasks to special repository providers.
Maven libraries should only be used in the repository provider implementations. All other modules should use the central repository api.