This is the Core codebase of Sonatype Nexus, aka "Nexus OSS".
To build this project you will need to add the Sonatype Forge repository to your setup:
https://repository.sonatype.org/content/groups/forge
If you are adding proxies for this in a Nexus instance be sure to add both release and snapshot proxy repositories for the above URL.
Have fun,
Sonatype Team