flua: Move to a new flua package

flua is a standalone third-party component that deserves its own
package. In particular, this means things can use flua without
having to depend on FreeBSD-utilities, which will be useful as
more base utilities use flua.

This saves ~500kB in FreeBSD-utilities for systems which don't
need flua.

MFC after: 3 days
Reviewed by: kevans
Sponsored by: https://www.patreon.com/bsdivy
Differential Revision: https://reviews.freebsd.org/D53161