Prepare for the path alias changes in Drupal 8.8

Problem/Motivation

Path aliases have been converted to entities in Drupal 8.8: https://www.drupal.org/node/3013865

Since, the path alias forms will also be converted to entity forms in #3007832: Convert custom path alias forms to entity forms, this module will need a few adjustments to in order to keep it working with the latest Drupal minor version.

Proposed resolution

Figure out what changes are needed, preferably in a way that keeps the module compatible with Drupal 8.7 and below as well.

Remaining tasks

TBD.

User interface changes

Nope.

API changes

TBD.

Data model changes

TBD.