Fix missing subpackages in pyproject.toml

[Guereak]

The current pyproject.toml only declares ["sam3", "sam3.model"] as packages, which causes
ModuleNotFoundError when importing from the library:

ModuleNotFoundError: No module named 'sam3.sam'

This PR updates the package discovery to include all required subpackages (sam3.sam, sam3.train,
sam3.model.utils, etc.) so that pip install works correctly.

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