There are cases where downstream projects want to overwrite upstream elements.
So far, either the downstream project just allows overlaps and hopes that ABI is compatible. There are also files left from original elements when not overwritten.
Or downstream adds an "overlay" with local source to the junctions. However on the latter, elements in the overlay are considered as part of upstream, which means in cannot depend on element outside of upstream (unless cyclic junction maybe?).
Here we add a feature to junctions to allow injecting elements from current project to junctions.
This is useful for example in the case of GNOME SDK needing to override GLib, GObjectIntrospection, libsoup, etc. from Freedesktop SDK.
The backported branch to 1.4 !1914 (closed) was tested on GNOME SDK: https://gitlab.gnome.org/GNOME/gnome-build-meta/-/merge_requests/658