gh-132859: Run debugger scripts in their own namespaces (#132860)

Run debugger scripts in their own namespaces

Previously scripts injected by `sys.remote_exec` were run with the
globals of the `__main__` module. Instead, run each injected script
with an empty set of globals. If someone really wants to use the
`__main__` module's namespace, they can always `import __main__`.