9700: fix: Remove the legacy macro scoping hack r=matklad a=jonas-schievink
This stops prepending `self::` to single-ident macro paths, resolving even legacy-scoped macros using the fixed-point algorithm. This is not correct, but a lot easier than fixing this properly (which involves pushing a new scope for every macro definition and invocation).
This allows resolution of macros from the prelude, fixing https://github.com/rust-analyzer/rust-analyzer/issues/9687.
Co-authored-by: Jonas Schievink <jonasschievink@gmail.com>
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