new param for page target, links. List of hyperlinked-to targets

List of hyperlinked-to targets. We use dependencies for this now.
But circular dependencies are an error while hyperlink-cycles should be OK.

So param page(links=[...]) feeds into traversable_specs (versus traversable_dependency_specs).

While I was looking at the tests, un-commented-in some commented-out assertions that had a done TODO.

Testing Done:
https://travis-ci.org/pantsbuild/pants/builds/39520576

Bugs closed: 253, 742

Reviewed at https://rbcommons.com/s/twitter/r/1242/