dune-project: convert authors into list form
dune-project: convert authors into list form

The dune documentation states: authors is a list of items. Convert
the single string into a list of space separated strings.

This change has an effect only if generate_opam_files is set to true.

Signed-off-by: Olaf Hering <ded022db89399a060a39da31922f173c6556c2d9@aepfle.de>

paulp/unison