Prevent showing methods from blanket impls of not available foreign traits to show up in the search results
Fixes https://github.com/rust-lang/rust/issues/115480.
In the case that the blanket impl trait is not available in the current crate, we prevent adding its methods in the search index.
Now how I found how to fix the issue: the `equivalent` method is not generated in the documentation pages but was still added to the search index. To render impls, we iterate over `cache.impls` so I took a look at how this was generated. Inside `formats/cache.rs`, we have `CacheBuilder::populate` where we push impls into `impls` but with this condition:
```rust
if cx.cache.traits.contains_key(&trait_did) {
```
I re-used this condition in `CacheBuilder::fold_item` to prevent this method from being added in `cache.search_index` or `cache.orphan_impl_items`.
PS: If you want to double-check if the added test works, just comment the code I added in `cache.rs` and it should fail.
r? ``@notriddle``
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