PKGBUILDer is one of the rare helper that supports building of split packages. However, it currently always installs all built subpackages, instead of installing only the necessary subpackages.

The makepkg --pkg option was very useful in this very situation but it has been removed in pacman/makepkg 5.0, so using pacman -U is the only alternative. This option might however come back in the near future, see this recent discussion on pacman-dev mailing list.

You can test this with the python-novaclient PKGBUILD, which builds the python-novaclient and python2-novaclient subpackages, and whose dependencies are also split packages.