A temporary holding path until the library is ready to release.
The cmd module, its imports and the README referred to the library as
github.com/grmrgecko/firewall while the root go.mod declared it as
github.com/grmrgecko/go-firewall, so its require/replace pair never named the
module it was replacing. Point all of them at the new path, which makes them
agree for the first time. The go-firewalld dependency shares the old prefix and
is unrelated; it is left alone.
