Object Model Realignments on the Merge Train Product Category

The following discussion from !51433 (merged) should be addressed:

• @fabiopitino started a discussion: (+2 comments)

  @shinya.maeda In one refactoring I'm working on (on the side) to solve exactly this product vs code mismatch I'm introducing MergeTrains namespace for models too. In there there will be:

  • move MergeTrain to MergeTrains::Car. A merge request on train does not represent the whole train but a single car.
  • introduce a (non-ActiveRecord) model class MergeTrains::Train that would represent the whole train. Currently we user MergeTrain class methods for that but this refactoring would make it more explicit.

  It's going to look something like this:

  class MergeTrains::Train
    attr_reader :project, :target_branch
  
    def initialize(project, target_branch)
      # ...
    end
  
    def refresh!
      # ...
    end
  
    def drop_merge_request(merge_request)
      # ...
    end
  
    def queue_id
      # ...
    end
  
    def first_car
      # ...
    end
  end

  The current MergeTrain class would be containing only methods related to the train car. With that we could rename a few things like MergeTrains::RefreshMergeRequestsService -> MergeTrains::RefreshMergeTrainService and MergeTrains::RefreshMergeRequestService -> MergeTrains::RefreshCarService.