Add initial code for KojiResolver class.
    
    This commit:
    
    - Adds KojiResolver class and KojiResolver tests.
    - Changes the GenericResolver and its subclasses to pass base_module_mmds
      instead of base_module_nsvc to get_buildrequired_modulemds. This is needed,
      because KojiResolver needs to access XMD section of base module.
    - Implements KojiResolver.get_buildrequired_modulemds to ask Koji for list of
      modules tagged in the Koji tag and return their modulemds.