Update the module to be compatible with domain 3.x

Problem/Motivation

The module is currently only compatible with the domain version 2.x according to the composer.json file and update errors encountered, it would be good to update it so that it's also compatible with version 3.x of the domain module.

Steps to reproduce

Have both this module and domain installed (2.x) and then try to update the domain module with: composer require 'drupal/domain:3.x-dev@dev' and then observe the composer error:

  Problem 1
    - drupal/domain_unique_path_alias is locked to version 1.0.1-beta5 and an update of this package was not requested.
    - drupal/domain_unique_path_alias 1.0.1-beta5 requires drupal/domain ^2.0@beta -> found drupal/domain[dev-2.0.x, 2.0.0-beta1, ..., 2.0.x-dev (alias of dev-2.0.x)] but it conflicts with your root composer.json require (3.x-dev@dev).

Use the option --with-all-dependencies (-W) to allow upgrades, downgrades and removals for packages currently locked to specific versions.

Proposed resolution

Update the module to be compatible with 3.x of domain, I’ve been using it locally and have had one of our testers test it on a staging environment after having aliased the domain module in composer like so "drupal/domain": "3.x-dev as 2.0.0-beta8" and it's been working which goes to show it might be a fairly straight forward update because there hasn’t been any errors spotted.