Disallow creating unnamend (= autogenerated names) indices with the PHP DDL API

[TimWolla]

Autogenerated names for database indices are pretty messy, because the legacy API did not handle them consistently depending on whether the INDEX was included in the initial CREATE TABLE (unnamed, implicitly named after the first column by MySQL) or added after the fact using ALTER TABLE (autogenerated by WoltLab Suite).

The PHP DDL API should require an explicit name for all newly created indices to ensure a consistent and explicit result going forward. It needs to be discussed how to handle existing packages without explicit names.

Related #4360