Restructure the code
    
    - Add an --env variable to change the environment used via the CLI
    - Drop the ProductCache object
    - Introduce the DistgitBugzillaSync object which allows dropping global
      variables to rely on attributes instead.
    - Drop the pkgdb compatibility layer that was once introduced, pkgdb is
      no longer a thing and it's time to move on
    
    This changes a little bit the order in which projects are updated, it
    used to be by products first (Fedora, Fedora EPEL...) then package,
    now it's the opposite: by package first then product.
    
    Signed-off-by: Pierre-Yves Chibon <pingou@pingoured.fr>