fix(python): set module-name in pyproject.toml to resolve maturin build error (lance-format#137)

## Summary
Explicitly set `module-name = "lance_graph._internal"` in
`pyproject.toml`.
This fixes a build error where maturin was looking for `_internal` as a
top-level module in the python source directory, instead of placing it
inside the `lance_graph` package.

## Test plan
- Verified locally that `maturin develop` works correctly with this
change.
- CI should pass.


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Author: beinan

python/pyproject.toml

@@ -38,6 +38,7 @@ classifiers = [
 
 [tool.maturin]
 python-source = "python"
+module-name = "lance_graph._internal"
 python-packages = ["lance_graph", "knowledge_graph"]
 manifest-path = "../crates/lance-graph-python/Cargo.toml"