Changing user password fails on Drupal 6 sites

Running the following command fails on Drupal 6 sites:

    $ drush upwd admin --password=test
    PHP Fatal error:  Call to undefined function Drush\User\user_load_by_name()

The fix is pretty simple - override the load_by_name method of the Drush User object for Drupal 6.  I went ahead and overrode the user_load_by_mail function as well, since that doesn't exist in Drupal 6 either.