Cannot find recipe provider with custom file

Following the [docs](https://github.com/googlefonts/gftools/blob/main/docs/gftools-builder/README.md#creating-your-own-recipe-provider), I'm trying to write my own recipe provider.

I've tried both `provider.py` and `provider/__init__.py`.

---

Reproducer:

`config.yaml`
```yml
recipeProvider: test
```

`test.py`
```py
from gftools.builder.recipeproviders import RecipeProviderBase

class TestProvider(RecipeProviderBase):
def write_recipe(self):
raise NotImplementedError("but it works")
```

Run: `uvx gftools builder config.yaml` (or set up & activate a venv more traditionally, same outcome)

---

Interestingly, it works when invoking `gftools.builder` under a debugger