Officially support copying of artifacts

[quintesse]

Right now I created the ModuleCopycat class that takes care of copying a list of artifacts with their set of suffixes to copy from one repository to another. That works kind of okay but has several problems I can think of:

• error handling: if the copy fails in the middle we are left with an inconsistent state in the cache
• the ArtifactCallback really doesn't show proper progress (it would be divided into several parts, like copying first from remote to local cache and then a second copy to the final destination)