The implementation files for the onnxscript converter should be moved into the _internal folder to clarify that these are not public and that users should not take a dependence on the implementation details.

This likely includes files such as converter.py, evaluator.py, irbuilder.py, main.py, type_annotation.py, and values.py, along with corresponding test files.

The main types intended to be made public should be imported from these files and made public by the init.py file.