Silence maturin missing python module _internal warning

[beinan]

Summary
During uv run --project . maturin develop, maturin warns that python/python/_internal is missing when python-source is python. The cdylib is named _internal, so maturin expects a module there.

Repro

cd python
uv run --project . maturin develop

Expected
Build completes without warnings.

Notes / Options

• Add python/_internal/__init__.py (empty) to satisfy maturin’s module expectation.
• Alternatively adjust python-source/python-packages or rename the cdylib, but the empty package is minimal.

Environment

• After moving Rust bindings to crates/lance-graph-python
• Python 3.11, maturin via uv