gh-126631: gh-137996: fix pre-loading of `__main__` (GH-135295)

gh-126631: gh-137996: fix pre-loading of `__main__`

The `main_path` parameter was renamed `init_main_from_name`, update the
forkserver code accordingly.  This was leading to slower startup times when people
were trying to preload the main module.

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Co-authored-by: Gregory P. Smith <greg@krypto.org>