Refactor the install code to be generic, so we only need to add the new
path in one place when adding a new file. Put the full path into the
filename to make this possible.

I've tried to mimic the final directory structure in the aport first
(e.g. $pkgdir/sbin/swapfile -> main/postmarketos-base/sbin/swapfile),
but that leads to conflicts as abuild only takes the filename for
checksums (conflict with etc/conf.d/swapfile):
https://gitlab.alpinelinux.org/alpine/abuild/-/issues/10013

We don't really need checksum verification for the files shipped in the
same directory (not downloaded), but nevertheless this is a bug. It
leads to confusing behavior and might be fixed by letting abuild demand
that there is only one source file with the same name, as I suggested in
the issue linked above. So let's avoid this altogether with the flat
file name.