Make MBSResolver._get_modules public
    
    For an issue I'm working on, I need to get koji_tag from the latest
    platform module. For local build, MBSResolver is used to get module from
    a local mbs database and remote MBS if local database does not have the
    module. On the other hand, for a MBS service, it uses DBResolver to
    query modules from connected database directly. So, my case mentioned
    has to ensure platform module is queried from MBS service database
    whatever current module is being built locally or in a MBS service.
    
    This patch makes MBSResolver._get_modules public so that it can be
    called dynamically based on resolver configuration. As a result, I can
    get a platform module in this way,
    
      resolver = module_build_service.resolver.GenericResolver.create(conf)
      platform_modules = resolver.get_modules(name=name, stream=stream)
      return platform_modules[0]['koji_tag']
    
    MBSResolver._get_module is changed accordingly.
    
    Signed-off-by: Chenxiong Qi <cqi@redhat.com>