Consider a submodule to support pathauto

I am trying to identify what should be done and in which module...
I scanned pathauto's code and it seems the only thing actually needed is the following code:

\Drupal::service('pathauto.generator')->updateEntityAlias($entity, 'update');

So, we could directly invoke this call with a proper if pathauto module exists then condition. The question already rose in #2872697-23: Stop saving an entity when it gets added to or removed from a group → by @berdir but it's not yet clear what should be done exactly since this might affect other custom or contrib modules we're not currently aware of.

I currently vote for a submodule for the pathauto implementation. It could event get installed by default as part of an upgrade if the module is enabled in the installation. This submodule could also work as a best-practice example for any other module to solve the same problem in its own context.

As mentioned in the other issue, it needs to be a standalone thing. We cannot privilege Pathauto over other contrib modules so it's best to leave any contrib module related code out of Group and put it into targeted submodules instead.