/usr/lib/mysql/plugin/handlersocket.so.0 is in handlersocket-mysql-5.5 1.1.0-7-g1044a28-2+b1.
This file is owned by root:root, with mode 0o777.
It is a symlink to /usr/lib/mysql/plugin/handlersocket.so.0.0.0
/usr/lib/mysql/plugin/handlersocket.so.0.0.0