Be able to depend on a junction element, by resolving its 'default targets'

[See original issue on GitLab](https://gitlab.com/BuildStream/buildstream/-/issues/1331)
In GitLab by [[Gitlab user @tpollard]](https://gitlab.com/tpollard) on May 28, 2020, 14:27

So if I give a project a default target (which is buildable), I can junction it into another project and `bst build $junction.bst` and resolve a build down to a specified element, or a given list. See https://buildstream.gitlab.io/buildstream/format_project.html?highlight=default%20target#default-targets

Would it be plausible to expect the same to happen if depending on a junction as a build dependency? Currently it doesn't work, as bst will complain as expected that you can't depend on a junction element as they're not build-able by definition. See https://gitlab.com/celduin/bsps/example-app/-/jobs/569617114

Start with the linked default-targets documentation and the junction dependency behavior described in the issue. Reproduce the failure with `bst build $junction.bst` and inspect the resolution path for build dependencies. Done means a junction used as a dependency resolves its default target or targets without being treated as directly buildable.

Written by the indexing model from the issue text.