gh-127960 Fix the REPL to set the correct namespace by setting the correct `__main__` module (gh-134275)

The `__main__` module imported in the `_pyrepl` module points to the `_pyrepl` module itself when the interpreter was launched without `-m` option and didn't execute a module,
while it's an unexpected behavior that `__main__` can be `_pyrepl` and relative imports such as `from . import *` works based on the `_pyrepl` module.

Co-authored-by: Łukasz Langa <lukasz@langa.pl>