fix: deprecations triggered by debug class loader

[shyim]

Fixes following errors:

21:05:05 INFO      [php] User Deprecated: Method "Composer\Plugin\PluginInterface::activate()" might add "void" as a native return type declaration in the future. Do the same in implementation "Symfony\Flex\Flex" now to avoid errors or add an explicit @return annotation to suppress this message.
21:05:05 INFO      [php] User Deprecated: Method "Composer\Plugin\PluginInterface::deactivate()" might add "void" as a native return type declaration in the future. Do the same in implementation "Symfony\Flex\Flex" now to avoid errors or add an explicit @return annotation to suppress this message.
21:05:05 INFO      [php] User Deprecated: Method "Composer\Plugin\PluginInterface::uninstall()" might add "void" as a native return type declaration in the future. Do the same in implementation "Symfony\Flex\Flex" now to avoid errors or add an explicit @return annotation to suppress this message.
21:05:05 INFO      [php] User Deprecated: Method "JsonSerializable::jsonSerialize()" might add "mixed" as a native return type declaration in the future. Do the same in implementation "Symfony\Flex\Response" now to avoid errors or add an explicit @return annotation to suppress this message.
21:05:05 INFO      [php] User Deprecated: Method "Composer\DependencyResolver\Operation\OperationInterface::getOperationType()" might add "string" as a native return type declaration in the future. Do the same in implementation "Symfony\Flex\InformationOperation" now to avoid errors or add an explicit @return annotation to suppress this message.
21:05:05 INFO      [php] User Deprecated: Method "Composer\DependencyResolver\Operation\OperationInterface::show()" might add "string" as a native return type declaration in the future. Do the same in implementation "Symfony\Flex\InformationOperation" now to avoid errors or add an explicit @return annotation to suppress this message.

[fabpot]

Thank you @shyim.