Rework `non_local_definitions` lint to only use a syntactic heuristic
This PR reworks the `non_local_definitions` lint to only use a syntactic heuristic, i.e. not use a type-system logic for whenever an `impl` is local or not.
Instead the new logic wanted by T-lang in https://github.com/rust-lang/rust/issues/126768#issuecomment-2192634762, which is to consider every paths in `Self` and `Trait` and to no longer use the type-system inference trick.
`@rustbot` labels +L-non_local_definitions
Fixes#126768
f5cd2c5888