Harden fallback class map generation
It could happen when an extension does not provide any
autoload definition, that valid classes are not or
invalid classes are taken into account during class map generation.
To fix this, update to latest composer code that supports
a blacklist for class map generation and provide tests
folder and ext_update files as valid black list.
Also add "Resources" and "res" folder to black list, which
very often also contain php classes, which are then manually required.
Also fix, streamline and add tests to verify this behavior.