Handle missing references on partial explorer snapshots

When saving a partial distribution and filtering on bundle prefix for instance, some selected references might not be present in the final distribution --> handle them and avoid displaying a link to a non-existing artifacts.

Steps to reproduce: save partial snapshot by filtering on bundle prefix "org.nuxeo.apidoc" like ftests are doing. The explorer package will be selected and will display that it contains the nuxeo-webengine-ui bundle, with a link, that leads to 404: the bundle has been filtered out.