Postoffice compat theme suggestions incompatible with OOP hooks

Problem/Motivation

Postoffice ships with several subextensions. Two of them seem to have an unfortunate name clash, which only surfaces under 11.1 and later (i.e., after the introduction of OOP hooks).

The module postoffice_compat defines theme suggestion hooks. E.g.:

/**
 * Implements hook_theme_suggestions_HOOK() for postoffice_compat_email_user.
 */
function postoffice_compat_theme_suggestions_postoffice_compat_email_user(array $variables) {

However, the new OOP hook discovery code attributes those functions to postoffice_compat_theme (longest common prefix) and thinks that the hook name is suggestions_postoffice_compat_email_user instead of theme_suggestions_postoffice_compat_email_user.

Steps to reproduce

Proposed resolution

Turn hook implementations into OOP hooks and add a compatibility shim (see CR.

Remaining tasks

User interface changes

API changes

Data model changes