The modulemd spec has a license: content: [list...] that should be filled out based on the license of all the binary rpms present in the given module.

@contyk suggests that this needs to be in pungi instead of in the MBS because pungi is the place where module filters are applied and we only want to list the licenses of binary rpms that are actually shipped (instead of the possibly larger set that includes unshipped rpms stripped out by filter mechanics).