Use composer for autoloads

[fiammybe]

Migrate our current legacy autoloader to a standards-compliant PSR-4 composer based setup.

• use composer instead of the legacy autoloader
• Implement fallback to old PHP5 class structure to have a well organised and planned deprecation, while keeping the modules and the core working through it all.
• Provide a bridge from the old PHP5 class structure to PSR-4 class structures that composer favors, enabling a smooth transition for modules and core code