pmb.helpers.pmaports.get_repo: "systemd" or None

This function is used by "pmbootstrap repo_missing", which is only used by bpo. In order to support building the split repository in bpo, this patch is required.

Return "systemd" for the packages that are in extra-repos/systemd, and None for all other packages (indicating that they don't get split into a separate repository).

Checking the parent-parent dir of the APKBUILD is a bit fragile (assuming we don't have subdirs in the systemd repository), but that assumption was made with the previous implementation as well (would only return "systemd" in that case). I don't see a better solution with reasonable effort right now, given that we also support multiple pmaports dirs. We can improve this in the future, meanwhile let's just not use subdirs in the systemd repository.