chore: update to maturin's recommended project layout for rust/python…

… projects

The previous layout leads to an import error when installing with `maturin build` and `pip install .`.

This error was common enough that `maturin` changed the recommended project layout to what this commit does.

A prior PR attempted to solve this by altering `lib.name` in Cargo.toml, but that did not work for me.

- [Prior PR](apache#694)
- [maturin ImportError issue](PyO3/maturin#490)
- [maturin changes recommended project structure](PyO3/maturin#855)

pyproject.toml

@@ -55,6 +55,7 @@ repository = "https://github.com/apache/arrow-datafusion-python"
 profile = "black"
 
 [tool.maturin]
+python-source = "python"
 module-name = "datafusion._internal"
 include = [
     { path = "Cargo.lock", format = "sdist" }