Auto merge of #30295 - jseyfried:fix_extern_crate_duplicate, r=nrc

Fix a bug allowing an item and an external crate to collide so long as the external crate is declared after the item. For example,
```rust
mod core { pub fn f() {} } // This would be an error if it followed the `extern crate`
extern crate core; // This declaration is shadowed by the preceding module

fn main() { core::f(); }
```
This is a [breaking-change], but it looks unlikely to cause breakage in practice, and any breakage can be fixed by removing colliding `extern crate` declarations, which are shadowed and hence unused.

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