REVIEW: [NX-163] remove plugin manager (step 2)

Second in a series of changes replacing the old plugin manager:

* follows on from PR #358
* replace `DefaultPlexusContainer` with `Guice.createInjector(modules)`

https://issues.sonatype.org/browse/NX-163
https://bamboo.zion.sonatype.com/browse/NX-OSSF12-4 :white_check_mark: