Squash-o-matic output

Squashes the Nexus Core modules into one "nexus-core" module. No Core code change, just shuffling them around.

Changes:
* nexus-core module introduced, with all the sources from following modules: nexus-utils, nexus-api, nexus-configuration, nexus-proxy, nexus-app and nexus-plugin-manager
* removed modules (well, "moved" them into):
* cleanup in UTs and ITs, mostly dependencies but some changes were done too, as suddenly the "mocked" environment became functional (many UTs from api, config or proxy were written by assuming Nexus Core is not functional there, as key code is added in app module).

CI went fine:
https://builds.sonatype.org/view/nexus-features/job/nexus-oss-its-feature/517/