Refactor "handlers" layout.
    
    This commit changes the directory structure for Freshmaker handlers
    as well as their names. The reason is that the current structure
    presumes there is just single handler for each Freshmaker Event,
    but this will not be true for new features we are working on.
    
    It is also not clear from the handler's name what is the goal of
    the handler which makes the code harder to understand.
    
    In this commit, we change the layout and names like this:
    
      - Handlers are now grouped according to the main service which
        they use when handling the Event - for example handlers
        resulting in Koji builds use 'koji' directory. Handlers updating
        just the Freshmaker's internal DB uses 'internal' directory, ...
      - Handlers are now named like DoSomethingOnSomeEvent. This makes
        it clear what is the goal of a handler.
    
    For more information, you can read:
    
    https://docs.google.com/document/d/1dx6cuFblaZlf0BZtPU6kv-ZzNJgf2JHWQ-VfSDuHVyM/edit#