## Summary
If a package in `site-packages` had this directory structure:
```py
# bar/__init__.py
from .a import A
# bar/a.py
class A: ...
```
then we would fail to resolve the `from .a import A` import _if_ (as is
usually the case!) the `site-packages` search path was located inside a
`.venv` directory that was a subdirectory of the project's first-party
search path. The reason for this is a bug in `file_to_module` in the
module resolver. In this loop, we would identify that
`/project_root/.venv/lib/python3.13/site-packages/foo/__init__.py` can
be turned into a path relative to the first-party search path
(`/project_root`):
6e2b8f9696/crates/red_knot_python_semantic/src/module_resolver/resolver.rs (L101-L110)
but we'd then try to turn the relative path
(.venv/lib/python3.13/site-packages/foo/__init__.py`) into a module
path, realise that it wasn't a valid module path... and therefore
immediately `break` out of the loop before trying any other search paths
(such as the `site-packages` search path).
This bug was originally reported on Discord by @MatthewMckee4.
## Test Plan
I added a unit test for `file_to_module` in `resolver.rs`, and an
integration test that shows we can now resolve the import correctly in
`infer.rs`.
a1eb834a5f