val field_names : Bos_setup.String.set
field_names is the maximal domain of the map returned by
fields, excluding extension fields (not yet supported by
val fields : dry_run:bool -> Fpath.t -> ( string list Bos_setup.String.map, Bos_setup.R.msg ) Bos_setup.result
fields f returns a simplified model of the fields of the opam file
f. The domain of the result is included in
field_names. Note that the
depopts: fields are returned without version constraints.