WIP: [NXCM-5327] Reimplement p2 composite repositories using linked (collection) items

Reimplement p2 composite repositories using linked items pointing to root of member p2 proxy repositories.

https://issues.sonatype.org/browse/NXCM-5327
https://bamboo.zion.sonatype.com/browse/NX-OSSF9

Includes changes from sonatype/nexus-oss/pull/54