Autodoc: only group structs under "namespaces"

The old heuristic of checking only for the number of fields has the
downside of classifying all opaque types, such as `std.c.FILE`, as
"namespaces" rather than "types".
2511830442Ian Johnson committed on 7/9/2024, 7:58:03 PM· committed by Andrew Kelleyparent854e86c
4 files changedLine totals unavailable