gh-132775: Drop PyUnstable_InterpreterState_GetMainModule() (gh-132978)

We replace it with _Py_GetMainModule(), and add _Py_CheckMainModule(), but both in the internal-only C-API.  We also add _PyImport_GetModulesRef(), which is the equivalent of _PyImport_GetModules(), but which increfs before the lock is released.

This is used by a later change related to pickle and handling __main__.