- Pass the media set to the Downloaders instead of having one in the object

- Pass the media set to the Downloaders instead of having one in the object

- Make Downloaders have a base class repo::Downloader
  this will allow to remove some code in RepoManager and interact
  with the downloader without knowing which type is it
- Make Downloader base class inherit from fetcher, then you can
  set the cache directories to the downloader itself, which
  RepoManager will do