Allow working with cross-junction elements

Today I'm bisecting a regression inside GnuTLS, which is part of the Freedesktop SDK. BuildStream does not allow me to directly build an element inside a junction, so I had to add my own GnuTLS element in gnome-build-meta that overlaps the GnuTLS provided by Freedesktop SDK. It would be much nicer if I could work with the element in the junction directly using a syntax like:

bst workspace open base/freedesktop-sdk-junction.bst:base/gnutls.bst ~/Projects/gnutls
bst build base/freedesktop-sdk-junction.bst:base/gnutls.bst
bst shell --build base/freedesktop-sdk-junction.bst:base/gnutls.bst

or something like that.

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