Auto merge of #112686 - estebank:sealed-traits, r=petrochenkov

Account for sealed traits in privacy and trait bound errors

On trait bound errors caused by super-traits, identify if the super-trait is publicly accessibly and if not, explain "sealed traits".

```
error[E0277]: the trait bound `S: Hidden` is not satisfied
  --> $DIR/sealed-trait-local.rs:17:20
   |
LL | impl a::Sealed for S {}
   |                    ^ the trait `Hidden` is not implemented for `S`
   |
note: required by a bound in `Sealed`
  --> $DIR/sealed-trait-local.rs:3:23
   |
LL |     pub trait Sealed: self::b::Hidden {
   |                       ^^^^^^^^^^^^^^^ required by this bound in `Sealed`
   = note: `Sealed` is a "sealed trait", because to implement it you also need to implelement `a::b::Hidden`, which is not accessible; this is usually done to force you to use one of the provided types that already implement it
```

Deduplicate privacy errors that point to the same path segment even if their deduplication span are different.

When encountering a path that is not reachable due to privacy constraints path segments other than the last, keep metadata for the last path segment's `Res` in order to look for alternative import paths for that item to suggest. If there are none, be explicit that the item is not accessible.

```
error[E0603]: module `b` is private
  --> $DIR/re-exported-trait.rs:11:9
   |
LL | impl a::b::Trait for S {}
   |         ^ private module
   |
note: the module `b` is defined here
  --> $DIR/re-exported-trait.rs:5:5
   |
LL |     mod b {
   |     ^^^^^
help: consider importing this trait through its public re-export instead
   |
LL | impl a::Trait for S {}
   |      ~~~~~~~~
```

```
error[E0603]: module `b` is private
  --> $DIR/private-trait.rs:8:9
   |
LL | impl a::b::Hidden for S {}
   |         ^  ------ trait `b` is not publicly reachable
   |         |
   |         private module
   |
note: the module `b` is defined here
  --> $DIR/private-trait.rs:2:5
   |
LL |     mod b {
   |     ^^^^^
```

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