postpone referencing sys.modules["__main__"]

[adriangb]

This currently breaks anyio (and presumably other things that do multiprocessing stuff) just from an import.
To reproduce:

from time import sleep

from anyio import to_process, run
import click  # this import breaks the program

async def main() -> None:
    await to_process.run_sync(sleep, 0)

if __name__ == "__main__":
    run(main)

cc @agronholm