/usr/share/php/Symfony/Component/Serializer/Mapping/Loader/ is in php-symfony-serializer 2.7.10-0ubuntu2.
This file is owned by root:root, with mode 0o755.
..
/usr/share/php/Symfony/Component/Serializer/Mapping/Loader/AnnotationLoader.php
/usr/share/php/Symfony/Component/Serializer/Mapping/Loader/FileLoader.php
/usr/share/php/Symfony/Component/Serializer/Mapping/Loader/LoaderChain.php
/usr/share/php/Symfony/Component/Serializer/Mapping/Loader/LoaderInterface.php
/usr/share/php/Symfony/Component/Serializer/Mapping/Loader/XmlFileLoader.php
/usr/share/php/Symfony/Component/Serializer/Mapping/Loader/YamlFileLoader.php
/usr/share/php/Symfony/Component/Serializer/Mapping/Loader/schema/
/usr/share/php/Symfony/Component/Serializer/Mapping/Loader/schema/dic/
/usr/share/php/Symfony/Component/Serializer/Mapping/Loader/schema/dic/serializer-mapping/
/usr/share/php/Symfony/Component/Serializer/Mapping/Loader/schema/dic/serializer-mapping/serializer-mapping-1.0.xsd