The existing license-file field is an alias to license-files,
restricted to only allow one license file to be specified (as is the
common case). Allowing multiple license files is necessary for some
licenses, such as the LGPL.
A previous patch introduced license-file field as an alias to
license-files, but this has the downside that the pretty printer will
print both. Instead, we now only have the license-files field and use
the UnrecParser mechanism to parse the license-file field.