8997: internal: stop expanding UseTrees during ItemTree lowering r=jonas-schievink a=jonas-schievink
Closes https://github.com/rust-analyzer/rust-analyzer/issues/8908
Messy diff, but `ItemTree` lowering got simpler, since we now have a strict 1-to-1 mapping between `ast::Item` and `ModItem`.
The most messy part is mapping a single `UseTree` back to its `ast::UseTree` counterpart for diagnostics, but I think the ad-hoc source map built during lowering does the job.
Co-authored-by: Jonas Schievink <jonasschievink@gmail.com>
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