/usr/share/php/Symfony/Component/Serializer/Mapping/ is in php-symfony-serializer 3.4.6+dfsg-1.
This file is owned by root:root, with mode 0o755.
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/usr/share/php/Symfony/Component/Serializer/Mapping/AttributeMetadata.php
/usr/share/php/Symfony/Component/Serializer/Mapping/AttributeMetadataInterface.php
/usr/share/php/Symfony/Component/Serializer/Mapping/ClassMetadata.php
/usr/share/php/Symfony/Component/Serializer/Mapping/ClassMetadataInterface.php
/usr/share/php/Symfony/Component/Serializer/Mapping/Factory/
/usr/share/php/Symfony/Component/Serializer/Mapping/Factory/CacheClassMetadataFactory.php
/usr/share/php/Symfony/Component/Serializer/Mapping/Factory/ClassMetadataFactory.php
/usr/share/php/Symfony/Component/Serializer/Mapping/Factory/ClassMetadataFactoryInterface.php
/usr/share/php/Symfony/Component/Serializer/Mapping/Factory/ClassResolverTrait.php
/usr/share/php/Symfony/Component/Serializer/Mapping/Loader/
/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