refactor: Move recipe generator to cook/ directory and fix flattening bugs

- Move recipe-generator/ to cook/ for cleaner structure

- Fix ksflatten-relative path conversion to handle all %include paths

- Fix validation exit code to only fail on actual errors

- All recipes now generate and flatten successfully via make all

cook/ingredients/section-data/post/base.ks

@@ -0,0 +1,9 @@
+# Core post-installation configuration
+
+%post --log=/mnt/sysimage/root/post.log # Beginning of the post-installation section. Log all messages to a given file
+
+localectl set-keymap ch-fr # Set keymap to `ch-fr`. Alternatively, `us` can be picked.
+dnf update -y # Update the system
+grub2-mkconfig -o /boot/grub2/grub.cfg # Unsure it is actually useful
+
+%end # End of the %post section