Add all build plug-in dependencies to the artifacts list whenever build plugins are validated

[cobratbq]

Add dependencies of build plug-ins as well as the build plug-ins themselves to the artifact validation list. (#54)

Given an earlier discovery that maven does not accept Version Range specification for build plug-in dependencies, it is trivial to add plug-in dependency artifacts: there is no need for version conflict resolution.

@slawekjaranowski let me know if you really want to add this under another configuration switch. Right now this would be included as soon as build plug-ins are included. (Personally, I would like to avoid it.)

[cobratbq]

I need to confirm if this dependencies list is a transitive closure (includes dependencies of dependencies). I think it is not. Even if it is not, this already adds value due to direct dependencies being verified, but in any case, it is good to understand the full scope of the validation.

[slawekjaranowski]

It will be good have at least one IT test to confirm it, or extends assertions in one existing test

[cobratbq]

You're right. I was finishing up when I found out that I missed something. I'll close it for now to avoid confusion.

[cobratbq]

@slawekjaranowski I've updated the IT test to check for plug-in dependencies too.

How do you feel about leaving TODO comments in the code where the implementation is not complete yet? (That is, complete as in, there is room to extend the functionality.) I'm hestitant to start extensive research on how Maven might perform dependency tree resolution for build plug-ins.
If you don't like it, how would you want to mark future work/room for improvement?