Don't touch extra-source-files in Parse.hs.

That was a typo.
Introduce an alternation combinator for UnrecParser, called 'composeUnrec'.
Have only a single 'license-files' field.

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.