/usr/include/debian-installer/ is in libdebian-installer4-dev 0.110ubuntu2.
This file is owned by root:root, with mode 0o755.
..
/usr/include/debian-installer/config.h
/usr/include/debian-installer/exec.h
/usr/include/debian-installer/hash.h
/usr/include/debian-installer/list.h
/usr/include/debian-installer/log.h
/usr/include/debian-installer/macros.h
/usr/include/debian-installer/mem.h
/usr/include/debian-installer/mem_chunk.h
/usr/include/debian-installer/package.h
/usr/include/debian-installer/packages.h
/usr/include/debian-installer/parser.h
/usr/include/debian-installer/parser_rfc822.h
/usr/include/debian-installer/release.h
/usr/include/debian-installer/slist.h
/usr/include/debian-installer/string.h
/usr/include/debian-installer/system/
/usr/include/debian-installer/system/devfs.h
/usr/include/debian-installer/system/dpkg.h
/usr/include/debian-installer/system/packages.h
/usr/include/debian-installer/system/prebaseconfig.h
/usr/include/debian-installer/system/subarch.h
/usr/include/debian-installer/system/utils.h
/usr/include/debian-installer/tree.h
/usr/include/debian-installer/types.h
/usr/include/debian-installer/utils.h