Draft: feat(release): Ship the 0BSD LICENSE file for packages sources + fail if it isn't present in the repo

Related to RFC 40: ship the 0BSD license file proper to Arch packages sources (in $DATADIR/packages) and fail if it isn't present in the repo when releasing a package.

EDIT: Marking this MR as "draft" while waiting for the defined grace period for NACK-ing the licensing to be over and for us to agree on a plan to move forward with the licensing itself.