This transition is now available when Person has no validated Login document.
Person's reference is now set only when it is not yet set.

Before it raised an exception if called with id=(A, B) where A has Login and B has no Login.

Simplified rework of ERP5 User Manager without module-level callables and
implementing more of PAS API.
Implement IAuthenticationPlugin API better, so PAS API becomes usable for
ERP5-based authentication.

use getId() instead of getUserName() where user is supposed to be neither Anonyous User nor System Processes.

so that getId() is used for ERP5 users and getUserName() is used for special users
like Anonyous User or System Processes whose id is None.

Allows being compatible with future, improved PAS plugin features.
Also, drop a few useless imports.

In ERP5User, getId() is the identifier, but for SpeicalUser, like
Anonymous User, System Processes etc., getId() is None.

otherwise self._person_value's path becomes like /erp5/acl_users/1
instead of /erp5/person_module/1.

Add methods to retrieve the user and login documents using PAS API.

For forward compatibility with multi-login, login-change,
login-is-not-id improvements.

This API is deprecated.