Do not parse collection 'link' as new OPF items

A new `OPFItem` was created for each `link` element. This PR makes
sure this does not happen with the new `link` element introduced as an
allowed child of the new EPUB 3.0.1 `collection` element.

This notably allows to **not** considered resources in a `collection` manifest as duplicates if already in the package manifest, as reported in issue #443