[ty] Improve lsp handling of hover/goto on imports (#21572)
* Fixes https://github.com/astral-sh/ty/issues/1011
* Also fixes the fact that we didn't handle `.x` properly *at all* in
hover/goto
It turns out all of our import handling completely ignored the `level`
(number of relative `.`'s) in a `from ..x.y import z` statement. It was
nice seeing how much my understanding of `ty` has improved -- previously
this would have all been opaque to me but now it was just, completely
glaring and blatant.
Fixing this required refactoring all the import code to take the
importing file into consideration. I ended up refactoring a bunch of
code to pass around/require `SemanticModel` more, as it's the natural
API for resolving this kind of import (it actually had an API for this
that was just... dead code, whoops!).
