Split WHENCE

Thank you for the recent split of this package. It is always nice to see a system take up less space after an update.

I however noticed, that the WHENCE file alone is still shipped in a dedicated package all of the linux-firmware-* depends upon. I though about programming up some lines that split it, too. The file precisely denotes which block applies to each file and as far as I can tell, there are no overlaps. So it should be possible to dissect it and compose an individual WHENCE for just the files included in each of the linux-firmware-* packages.

I know that WHENCE does not take up a very significant amount of space and this is no functional priority. But I like to clean things up, so I would take a stab at this. Would you be interested in a merge request? Or do you prefer WHENCE to be left in one piece?