Implement some way to distinguish packages that are already libre

[reback00]

I think we should distinguish the packages that are already libre on Void's repo in a different manner. Because we are building packages for our repo only seeing them in the allowlist.txt. But we don't need to build all packages from there. Especially those that are already libre on Void's repo. So we need something to distinguish between them and non-free packages.

Maybe adding their name after a : (similar to the original blacklist.txt) be a good idea to distinguish them:

<package to be allowed>:<package in the Void's default repo that's libre>

Second value would be optional. The packages that we need to build would have the second value blank. (Thus saying there are no options on void but to build it.)

If we make allowlist.txt like this:

libxfce4ui:libxfce4ui
minitube
grub
p7zip
unzip
acpi_call-dkms:acpi_call-dkms

Then we can run awk -F : '{ if ( $2 == "" ) { print $1 } }' allowlist.txt to see which packages we need to build:

minitube
grub
p7zip
unzip

This removes libxfce4ui and acpi_call-dkms because they're already libre and shows the actual packages that we need to build.