NEXUS-6185: Timestamped M2 Snapshots should be consumable over M1 shadow

Originally was
https://github.com/sonatype/nexus-oss/pull/291

Taken the two commits from that PR above, applied comments and formatting to it.

Issue
https://issues.sonatype.org/browse/NEXUS-6185

CI
TBD

This PR replaces the original one
https://github.com/sonatype/nexus-oss/pull/291