Ensure there are no duplicates in Elements.dependencies()

When we are not recursing, Element.dependencies() uses a much more light weight codepath since it just needs to print the direct dependencies. However, this simple codepath was not accounting for duplicates, in case something is both a build time and run time dependency.

One way this manifested itself was in bst show --format %{deps}, but it would also affect anything that was using this method to iterate on the dependencies.

Fixes #1308 (closed).