`from .a import b` in an `__init__.pyi` could also be a re-export of `b`

[Gankra]

This is a followup to astral-sh/ruff#21173 and pertains to behaviours described in imports/nonstandard_conventions.md

Currently we consider from . import b in an __init__.pyi a re-export idiom (equivalent to from . import b as b).

We could generalize this to from .a import b as well, although that's a bit less explicit.