opaque types in the defining scope auto trait candidate

[lcnr]

Due to auto trait leakage, we have an auto trait candidate for opaque types. Conceptionally auto trait leakage emulates assemble_candidates_after_normalizing_self_ty for things we can't normalize in a regular way. If we are able to normalize sensibly then we don't need to add auto traits for projections, this has been implemented in rust-lang/rust#112825.