The symlink

/usr/lib/libmysqlclient.so -> /usr/lib/libmysqlclient.so.18

was invalid, because the library was not in /usr/lib.
The correct symlink is

/usr/lib/x86_64-linux-gnu/libmysqlclient.so -> /usr/lib/x86_64-linux-gnu/libmysqlclient.so.18