Don't have one APKINDEX for each aports directory anymore

I thought this was a good idea, but seeing now how the amount of APKINDEX files explodes, I am not convinced of this anymore. I think it would be easier to stuff everything into one APKINDEX, at least for now.

The reasoning behind this was, to make it easy to ban firmware files from a system, if the user wants that. Maybe a nicer way to do that would generating a second APKINDEX, that has all packages except for the firmware packages. And then use that instead of the regular postmarketOS APKINDEX. But that can be done another time, and does not have priority.

@MartijnBraam: can you take care of this? That would reduce the amount of APKINDEX files it downloads a lot.