RFC to require `impl MyStruct` to be nearby the definition of `MyStruct`

This fixes #15060 and #3785.
There is no need for this restriction. The appropriate place to do the check is in `Resolver::resolve_implementation()`, which is called from `Resolver::resolve_crate()`, the final step of resolve. At this point, `struct MyStruct` will have been seen no matter where in the source file it occurs.