/usr/lib/x86_64-linux-gnu/libnlopt.so is in libnlopt-dev 2.4.2+dfsg-2+b2.
This file is owned by root:root, with mode 0o777.
It is a symlink to /usr/lib/x86_64-linux-gnu/libnlopt.so.0.8.2
/usr/lib/x86_64-linux-gnu/libnlopt.so.0.8.2