Rollup merge of #128932 - bvanjoi:issue-128813, r=petrochenkov

skip updating when external binding is existed

Fixes #128813

For following code:

```rs
extern crate core;

fn f() {
    use ::core;
}

macro_rules! m {
    () => {
        extern crate std as core;
    };
}

m!();

fn main() {}
```

- In the first loop, we define `extern crate core` and `use ::core` will be referred to `core` (yes, it does not consider if there are some macros that are not expanded. Ideally, this should be delayed if there are some unexpanded macros in the root, but I didn't change it like that because it seems like a huge change).
- Then `m` is expanded, which makes `extern_prelude('core')` return `std` rather than `core`, causing the inconsistency.

r? `@petrochenkov`

0 files changed, 0 insertions, 0 deletions