Refactor _password_policy_constraints_validate().

Looks good!

This code (which you didn't change so far) is mystifying to me:

  $expiration = $form_state->getValue('field_password_expiration');
  if (!is_null($expiration) && $expiration['value'] === FALSE) {
    $form_state->setValue('field_password_expiration', ['value' => 0]);
  }

I'm not sure what that has to do with validation. It looks like it's turning a FALSE (checkbox unchecked) into a 0 for storage in the database or something. Seems like maybe it should be a submit handler thing, and not in the _password_policy_constraints_validate(), but maybe I am missing the purpose. I'll git blame it later and see if that yields any clues.

Unsure why it's failing tests, but this looks terrific!

Hope to review/test this soon. Sorry for the delay, I had a problem with my Drupal development VM which I've now fixed.

Updated patch to apply on latest code.

Sorry for the extreme delay on reviewing/testing this.

Bad patch in #18. Working on it.

This patch fixes some minor issues:
- Remove unused variable $role_key.
- Change some function comments to third-person singular per documentation standards.
- Remove erroneous @return documentation.
- Remove changes unrelated to this issue (e.g., reordering use statements).

Note that interdiff versus #16 is not a pure interdiff because I first changed #16 to apply to latest -dev.

Couple more minor improvements:
- Add some missing function documentation.
- Improve variable name.

Forgot to change variable name in a place in #21. Trying again.

Committing without review/testing of #23, since it's minor changes versus #16 and the module is still in alpha.