de-duplicate overlapping POM elements in inheritance hierarchy

[jdcasey]

This now amounts to an enhancement, but it's always been broken.

MavenPomView should produce things like DependencyView that are collapsed where they overlap. For instance, if the parent declares a direct dep on junit with a scope of test, and a child POM redeclares the direct dep with scope of compile, the result of MavenPomView.getAllDirectDependencies() should be a single DependencyView with scope of compile.