Log in with email address only
It seems Login with Email only should solve that. At least worth a look how they managed to do that.


  This simple Drupal 8 module allows users to log in exclusively with
  their email address from the standard login form.
  
  Setup:
  
  Just enable the module.
  
  Recommended Modules:
  
  This module is based on the Login Email or Username module:
  https://www.drupal.org/project/login_emailusername