Decoupled users can't be referenced from entity reference element

Unfortunately, core's entity reference elements cannot find decoupled users because the autocomplete searches by username, even though it displays the display name.

web/core/modules/user/src/Plugin/EntityReferenceSelection/UserSelection.php uses the username field to search for autocomplete matches:

  protected function buildEntityQuery
...
    if (isset($match)) {
      $query->condition('name', $match, $match_operator);
    }

The only generic solution I can see is to store the display name in a field on the user entity, and then alter the entity reference query (hopefully with a hook, or an EntityReferenceSelection plugin if no hook suits) to filter by this new field instead of the username field.

This would also have the advantage of providing a relatively good solution out of the box for the problem of the name field on Views that I reported in #2630356: Make sure user views are not impacted by decoupling.