Ruff's first party import detection treats any folder in root as a package

Repro:
```
mkdir ruff_isort_repro
cd ruff_isort_repro
python -m venv .venv
. .venv/bin/activate
pip install ruff

mkdir schema
echo '{}' > schema/example-json-schema.json

mkdir example_package
touch example_package/sub.py
echo $'import schema\n\nfrom example_package import sub' > example_package/__init__.py
```
```
$ ruff . --select I001 --fix --diff
--- example_package/__init__.py
+++ example_package/__init__.py
@@ -1,3 +1,2 @@
import schema
-
from example_package import sub

Would fix 1 error(s).
```

I've run into this because I use a `schema` package from PyPI *and* I have a `schema` folder with JSON schemas within the root of the repository which causes ruff to think that `schema` is a package. I think that one way of solving this would be to alter the current heuristic to check for `{base}/__init__.py` file and `{base}.py` file rather than `{base}` directory and `{base.py}` file. This may potentially affect namespace packages and I'm not sure there's a good solution for this that doesn't involve explicitly specifying namespace packages.

Reproduce the report with the ruff commands and the generated example_package/__init__.py, then trace Ruff's first-party import detection and its handling of the schema directory. Done means the PyPI-style schema import is not classified as first-party while example_package remains correctly classified, without regressing namespace-package behavior.

Written by the indexing model from the issue text.