Rollup merge of #40509 - jseyfried:duplicate_check_macro_exports, r=nrc

Rollup merge of #40509 - jseyfried:duplicate_check_macro_exports, r=nrc

Forbid conflicts between macros 1.0 exports and macros 2.0 exports

This PR forbids for conflicts between `#[macro_export]`/`#[macro_reexport]` macro exports and `pub use` macro exports. For example,
```rust
// crate A:
pub use macros::foo;
//^ This is allowed today, will be forbidden by this PR.

// crate B:
extern crate A; // This triggers a confusing error today.
use A::foo; // This could refer to refer to either macro export in crate A.
```

r? @nrc