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)

Debug memory block at address p=0000000002EB8B90: API ' '
    15987178197214944733 bytes originally requested
    The 7 pad bytes at p-7 are not all FORBIDDENBYTE (0xfd):
        at p-7: 0x00 *** OUCH
        at p-6: 0x00 *** OUCH
        at p-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.