As mentioned in official resource:
The file /etc/packages.conf in the initramfs ...
initramfs file you can get in /boot directory.
It will be named as initramfs... or initrd....
You can inspect this file with lsinitramfs or lsinitrd or even extact over cpio command. But you need to ensure if the initramfs is compressed or not via file <initramfs_file> command. If it will be zipped/gziped you need to use zcat instead of cat.
So create some directory (<your_directory>) and copy initramfs to it initramfs file. Enter to <your_directory> and (considering compression) execute:
cat <initramfs_file> | cpio -idmv .
or:
zcat <initramfs_file> | cpio -idmv .
Finally you can get packages.conf in <your_directory>/etc/packages.conf.