Remove use of new DebugClassLoader functionality

Fixes a bug introduced in 427e024ab1da1ae7e69564a94fd25166d4456d6d - it relies on symfony/symfony#d10ad0f9e71d8ea8566e379192263a9aaba0b060 which isn't available in all versions (while this bundle advertises it works with any symfony). In any case reflection is much more reliable than adding a dependency on a particular class loader.