Auto merge of #57199 - petrochenkov:ambig, r=estebank

resolve: Simplify treatment of ambiguity errors

If we have a glob conflict like this
```rust
mod m1 { struct S; }
mod m2 { struct S; }

use m1::*;
use m2::*;
```
we treat it as a special "ambiguity item" that's not an error by itself, but produces an error when actually used.
```rust
use m1::*; // primary
use m2::*; // secondary

=>

ambiguity S(m1::S, m2::S);
```

Ambiguity items were *sometimes* treated as their primary items for error recovery, but pretty irregularly.

After this PR they are always treated as their primary items, except that
- If an ambiguity item is marked as used, then it still produces an error.
- Ambiguity items are still filtered away when exported to other crates (which is also a use in some sense).

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