Add support for defining target for junction elements
Add support for specifying target of a junction element to be a sub-sub-project as a configuration option. When such a target is defined, the junction element is not allowed to have any sources. This was discussed in the following mailing list thread: https://mail.gnome.org/archives/buildstream-list/2019-April/msg00025.html Since this adds new configuration, also bump `BST_FORMAT_VERSION`.
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