## Summary

`map_stub_definition` maps a stub definition to its "real-definition"
(the same definition in a `py` file. This PR extends this mapping by
resolving aliases. For:

`main.py`:

```py
from a import bar

bar
```

`a/__init__.pyi`:

```py
def bar() -> None: ...
```

`a/__init__.py`:

```py
from .b import bar as bar

```

`a/b.py`:

```py
def bar() -> None:
    """Implementation docstring"""
```

[Playground](https://play.ty.dev/f7255368-b291-4865-b4ca-6cac35a437e8)

Clicking on `bar` in `main.py` performs the following step:

1. Resolve `bar` to `from a import bar` in `main.py`
2. Follow the import to `a/__init__.pyi` where it finds `def bar()`
3. `map_stub_definition` now tries to map `bar` to its implementation.
It finds the `from .b import bar as bar` in `a/__init__.py` and **stops
here**


The step that is missing, and this PR adds, is that stub mapping must
follow the import to the symbol's real definition.


The same applies when resolving documentation: ty first tries to get the
documentation from the stub. If that's missing, ty tries to resolve the
documentation from the implementation. However, that requires resolving
the import, because the import itself has no docstring.


Closes astral-sh/ty#2150