Improvements to ModuleIndex.update_from_defaults_directory()
    
      * Import each file in the directory as a merge rather than an overwrite so
        we can detect conflicts.
      * Modify the meaning of the 'strict' argument to fail if the merge would
        result in a conflict in the default stream setting of a module.
    
    Signed-off-by: Stephen Gallagher <sgallagh@redhat.com>