/usr/include/core/ is in libnet-cpp-dev 2.0.0-0ubuntu1.
This file is owned by root:root, with mode 0o755.
..
/usr/include/core/location.h
/usr/include/core/net/
/usr/include/core/net/error.h
/usr/include/core/net/http/
/usr/include/core/net/http/client.h
/usr/include/core/net/http/content_type.h
/usr/include/core/net/http/error.h
/usr/include/core/net/http/header.h
/usr/include/core/net/http/method.h
/usr/include/core/net/http/request.h
/usr/include/core/net/http/response.h
/usr/include/core/net/http/status.h
/usr/include/core/net/http/streaming_client.h
/usr/include/core/net/http/streaming_request.h
/usr/include/core/net/uri.h
/usr/include/core/net/visibility.h