/usr/lib/x86_64-linux-gnu/libmetacity-private.so is in libmetacity-dev 1:3.18.3-1ubuntu3.
This file is owned by root:root, with mode 0o777.
It is a symlink to /usr/lib/x86_64-linux-gnu/libmetacity-private.so.3.0.0
/usr/lib/x86_64-linux-gnu/libmetacity-private.so.3.0.0