Simplify adding services to systemd-services package

Feedback from !5377 (comment 2060619811) ...

@ollieparanoid suggested that maybe it could be simplified by doing something like:

• bump pkgrel
• add the service file in a path that also encodes the file name, e.g. user/modemmanager-systemd/ModemManager.service
• update checksums

The APKBUILD would have something like this, untested:

_services="$(find user system -name '*.service' -o -name '*.socket')"
sources="$_services"
subpackages=""
for i in $(echo "$_services" | cut -d / -f 2 | sort -u); do
  subpackages="$subpackages $i:_service"
done

This would make it much more elegant to add a service. We would need a pmbootstrap patch though that lets it know that *-systemd packages are likely to come from systemd-services if it cannot be found otherwise. @craftyguy isn't sure what the pmbootstrap patch would be exactly, but assumes @ollieparanoid will elaborate if we decide this is the best approach to simplify this package's APKBUILD