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>

Lib/_pyrepl/main.py

@@ -1,6 +1,7 @@
 import errno
 import os
 import sys
+import types
 
 
 CAN_USE_PYREPL: bool
@@ -29,12 +30,10 @@ def interactive_console(mainmodule=None, quiet=False, pythonstartup=False):
             print(FAIL_REASON, file=sys.stderr)
         return sys._baserepl()
 
-    if mainmodule:
-        namespace = mainmodule.__dict__
-    else:
-        import __main__
-        namespace = __main__.__dict__
-        namespace.pop("__pyrepl_interactive_console", None)
+    if not mainmodule:
+        mainmodule = types.ModuleType("__main__")
+
+    namespace = mainmodule.__dict__
 
     # sys._baserepl() above does this internally, we do it here
     startup_path = os.getenv("PYTHONSTARTUP")