/usr/lib/x86_64-linux-gnu/libgtksourceview-3.0.so.1 is in libgtksourceview-3.0-1 3.18.2-1.
This file is owned by root:root, with mode 0o777.
It is a symlink to /usr/lib/x86_64-linux-gnu/libgtksourceview-3.0.so.1.5.0
/usr/lib/x86_64-linux-gnu/libgtksourceview-3.0.so.1.5.0