De-duplicate plugin views just like we did with dependencies

[jdcasey]

Need to verify this is a general-case problem:

https://github.com/jdcasey/pom-manipulation-ext/pull/68

[jdcasey]

maven will collapse these overlaps by merging plugin configurations. We need to look at doing the same thing.

[rnc]

Updated link to PME : project-ncl/pom-manipulation-ext#68

Might also be related to #96