NEXUS-5884 Remove use of Nexus god object

Disassemble Nexus god object. Almost all of the method implementations in DefaultNexus were one liners calling into some other component.

All methods are now deprecated, all code is "redirected" to proper places.

Issue
https://issues.sonatype.org/browse/NEXUS-5884

CI
https://bamboo.zion.sonatype.com/browse/NX-OSSF52