Project.refs format unclear when multiple sources used per element

We bump during freedesktop-sdk/freedesktop-sdk!1576 (diffs) to the issue that project.refs is quite unclear with multiple sources per element since there's insufficient context available to understand what each ref means. This makes reviewing changes quite hard when project refs are used and as a result freedesktop-sdk didn't take project.refs into use and gnome-build-meta is considering dropping project.refs.

The underlying idea of splitting the data that BuildStream actively modifies during track into a separate file is reasonable but current design doesn't seem to be sufficient for BuildStream projects.