/usr/bin/texliveonfly is in texlive-extra-utils 2017.20180305-2.
This file is owned by root:root, with mode 0o777.
It is a symlink to /usr/bin/../share/texlive/texmf-dist/scripts/texliveonfly/texliveonfly.py
/usr/bin/../share/texlive/texmf-dist/scripts/texliveonfly/texliveonfly.py