ImportMojo should mark the direct XAR dependencies as explicitly installed

This causes for instance the distribution UI (xwiki-enterprise-ui-mainwiki) to be marked as "Installed as dependency" although there isn't any other extension that requires it (i.e. the distribution UI has no backward dependencies). I think the direct XAR dependencies that are imported and installed by the ImportMojo should be marked as explicitly installed while their transitive dependencies should be marked as "Installed as dependency".