gh-117953: Always Run Extension Init Func in Main Interpreter First

[ericsnowcurrently]

• Issue: Single-Phase Init Extension Module Init Functions Still Run in Isolated Interpreters #117953

[neonene]

I tried this PR on Windows. The following two results were unexpected to me:

...
>>>_xxsubinterpreters.run_string(i, "import _datetime; print(_datetime)")
<module'_datetime' (built-in)># legacy module is imported?

...
>>>_xxsubinterpreters.run_string(i, "import _testcapi")
>>>_xxsubinterpreters.destroy(i)
Debugmemoryblockataddressp=0000000002EB8B90: API' '15987178197214944733bytesoriginallyrequestedThe7padbytesatp-7arenotallFORBIDDENBYTE (0xfd):
atp-7: 0x00***OUCHatp-6: 0x00***OUCHatp-5: 0x00***OUCH
...

[ericsnowcurrently]

Thanks for trying it. That info is helpful. FYI, I'm working on extra changes here as the current PR appears to be incomplete.