Fail to find implemented trait in the list returned from `all_trait_decls`

[celinval]

This could be a naming issue, but I expect all_trait_decls and all_trait_impls to return all traits declarations and implementations available, i.e.:, all local and external traits.

Maybe we can rename these functions to local_trait_decls and local_trait_impls. In this case, should we also provide an API that returns trait declarations and implementations from external crates.

[celinval]

@rustbot claim