Import Original and Supporting files separately

This change modifies the import side to assume that the export side
knows which files are originals and which are supporting and exports
them as two distinct columns.

By defaulting the supporting_file_paths argument in Base#initialize to,
we can continue to use existing exports with the only caveat being
everything's assumed original.

Also, in accordance with Wendy's advice, we now parse works from each
original file in turn and stop on the first file to parse successfully.