* flower/file-path.cc (find): don't throw away file_name.dir, but

* flower/file-path.cc (find): don't throw away file_name.dir, but
append to it. Fixes \include with directories.

* flower/include/file-path.hh (class File_path): don't derive from
Array<String>.

* flower/include/file-name.hh (class File_name): remove to_str0()

* lily/hara-kiri-engraver.cc (acknowledge_grob): split
Hara_kiri_engraver in separate file.