REVIEW: [NX-163] remove plugin manager (step 1)

First in a series of changes replacing the old plugin manager:

* register static resources in plugin module
* use mediator to register repository types
* trim+remove various redundant classes

https://issues.sonatype.org/browse/NX-163
https://bamboo.zion.sonatype.com/browse/NX-OSSF4-3 :white_check_mark: