/usr/lib/x86_64-linux-gnu/mysqlclient.pc is in libmariadbclient-dev-compat 1:10.1.29-6.
This file is owned by root:root, with mode 0o777.
It is a symlink to /usr/lib/x86_64-linux-gnu/pkgconfig/mariadb.pc
/usr/lib/x86_64-linux-gnu/pkgconfig/mariadb.pc