[Extensions] Consider contracts

[Guite]

Symfony uses contracts for some interfaces, like Symfony\Contracts\Translation\TranslatorInterface. This has the advantage that component A can implement and use it without requiring component B.

Proposal:

• introduce a Zikula extension contracts package
• use this for interfaces and base classes from extensions like Content and MultiHook which currently live in Zikula\ExtensionsModule\ModuleInterface
• move Block handler interface and base class into that contracts namespace
• move Search helper interface and base class into that contracts namespace
• outsource Search module and re-include it as VA extension (like Legal, OAuth, PageLock, Profile) to make having a site search optional again

cc @gfr