When using PythonFunction, current implementation generates handler value incorrectly.

Code

new PythonFunction(stack, 'handler', {
    entry: 'test/lambda-handler-sub-nested',
    index: 'inner/inner2/custom_index.py',
    handler: 'custom_handler',
    runtime: Runtime.PYTHON_3_8,
});

Expected

Handler: inner.inner2.custom_index.custom_handler

Current Behaviour

Handler: inner.inner2/custom_index.custom_handler

This PR fix the issue by modifying string replace pattern.

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