Auto merge of #66078 - petrochenkov:gateout, r=Centril

expand: Feature gate out-of-line modules in proc macro input

Extracted from https://github.com/rust-lang/rust/pull/64273.

We are currently gating attributes applied directly to `mod` items because there are unresolved questions about out-of-line modules and their behavior is very likely to change.

However, you can sneak an out-of-line module into an attribute macro input using modules nested into other items like
```rust
#[my_attr]
fn m() {
    #[path = "zzz.rs"]
    mod n; // what tokens does the `my_attr` macro see?
}
```
This PR prevents that and emits a feature gate error for this case as well.

r? @Centril
It would be great to land this before beta.

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