Summary displays don't work on attachment displays added to EVA displays

This could be handled in EVA by adding DisplayRouterInterface.

WIP patch that changes Eva to extend PathPluginBase instead of DisplayPluginBase.

While #4 makes it possible to build a glossary display without a WSOD, the URLs on the links are wrong.

The patch above didn't show any output; in my efforts to fix the path problems resulted in my uploading the wrong changes.

This shows the output, but though the path problem still exists.

FWIW here's the JS code I used with the patch above to make the display load via AJAX, because the standard Views AJAX logic wasn't being triggered:

(function ($, Drupal, drupalSettings) {
  'use strict';

  /**
   */
  Drupal.behaviors.LandingPageViewBySubject = {
    attach: function (context, settings) {
      $('.PARENTCLASS .view .views-summary > a', context).on('click', function(e) {
        e.preventDefault();
        var the_letter = this.innerText;
        var the_context = context;
        $.ajax({
         'url': Drupal.url('views/ajax'),
         'type': 'POST',
         'data': {
           'view_name': 'VIEWNAME',
           'view_display_id': 'DISPLAYNAME',
           'view_args': the_letter,
          },
         'dataType': 'json',
         'async': false,
         'success': function (response)  {
           if (response[3] !== undefined) {
             var viewHtml = response[3].data;
             // Remove previous results and add the new ones.
             $('. PARENTCLASS > .views-element-container').html('');
             $('. PARENTCLASS > .views-element-container').append(viewHtml);
    
             // Attach Latest settings to the behaviours and settings. 
             // it will prevent the ajax pager issue
             Drupal.settings = response[0].settings;
             drupalSettings.views = response[0].settings.views;
             Drupal.attachBehaviors(the_context, Drupal.settings);
           }
         }
        });
        return false;
      });
    }
  };
})(jQuery, Drupal, drupalSettings);

(adjust PARENTCLASS, VIEWNAME and DISPLAYNAME as appropriate.