Rather than merging subSelectionSets of a field set using MergeSelectionSets and then calling CollectFields, introducing CollectSubfields allows the field set's groupedSubfieldSet to be calculated directly.

This may be helpful if the specification were ever to be altered such that additional state beyond the current selection set were to be required to calculate the response, i.e. if it were to be required to know the originating selectionSet of a given field within the fieldSet for determining when to communicate a reference signal.

See graphql#998 (comment)

Remove unnecessary return value change

Co-authored-by: Benjie <[email protected]>