Support adding Editions to Works as required_resources

[pmakholm]

When scraping a work for a site, I would like to add (selected) editions.

I assumed that I could just return editions in pd.metadata["required_resources"]. This works correctly when I add a work when scraping editions.

Looking at the code this is due to a missing Work.update_linked_items_from_external_resource() method. This commits naively copies the implementation from Edition.update_linked_items_from_external_resource().

[alphatownsman]

Thanks, merged this. However, I suggest putting edition info in related_resources rather than required_resources in your work.scrape() code