Rollup merge of #80845 - GuillaumeGomez:item-kind-transition, r=jyn514

Make ItemKind::ExternCrate looks like hir::ItemKind::ExternCrate to make transition over hir::ItemKind simpler

It was surprisingly difficult to make this change, mostly because of two issues:

* We now store the `ExternCrate` name in the parent struct (`clean::Item`), which forced me to modify the json conversion code a bit more than expected.
* The second problem was that, since we now have a `Some(name)`, it was trying to render it, ending up in a panic because we ended up in a `unreachable` statement. The solution was simply to add `!item.is_extern_crate()` in `formats::renderer` before calling `cx.item(item, &cache)?;`.

I'll continue to replace all the `clean::ItemKind` variants one by one until it looks exactly like `hir::ItemKind`. Then we'll simply discard the rustdoc type. Once this done, we'll be able to discard `clean::Item` too to use `hir::Item`.

r? ``@jyn514``

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