generator: Rename --gir-directory to --gir-files
generator: Rename --gir-directory to --gir-files

gir-directory mentally conflicts with the directory for `gir`, rather
than the specifically-named gir-files directory.

GuillaumeGomez/gir,gtk-rs/gir,gtk-rs/gir,GuillaumeGomez/gir,gtk-rs/gir,GuillaumeGomez/gir