Ship libmakepkg-dropins with 7.1.0

For #42 and linux#2 (closed) (linux!3 (merged)) we need the plugins from libmakepkg-dropins to be available in makepkg. The 7.1.0 release of this package should include these, which will probably involve adding git+https://gitlab.archlinux.org/pacman/libmakepkg-dropins.git#commit=<commit> as a source.