rustdoc: blanket impl does not work for re-exported items

The new "blanket implementations" does not seem to work if an item is re-exported in a different location.  For example:

```rust
pub struct S2 {}
mod m {
    pub struct S {}
}
pub use m::*;
```

`S2` shows the blanket implementations, but `S` does not.

Tested with rustc 1.30.0-nightly (d5a448b3f 2018-08-13).

cc @GuillaumeGomez