Allow creation of aggregated javadocs source bundles from project dependencies

It would be nice to have the ability to generate an aggregated javadoc set for a distribution project by resolving the -sources and -test-sources bundles of its dependencies (or, correspondingly, the project.compileSourceRoots and project.testCompileSourceRoots for modules in the same reactor).

Initially, this might just mean downloading, unpacking, and adding the dependency sources as sourcepaths to the javadoc execution if a flag is set to true (includeDependencySources). Later, we could easily expand this to allow bundling and deployment of the src/main/javadoc directory so that this artifact can be used in the above aggregation approach.

I've got an implementation of the first part that I will attach to this issue as a patch to illustrate what I'm talking about.