The Python documentation [1] says that `PyImport_AppendInittab` should
be called before `Py_Initialize()`. Starting with Python 3.12, this is
enforced with a fatal error:
Fatal Python error: PyImport_AppendInittab: PyImport_AppendInittab()
may not be called after Py_Initialize()
This commit ensures we only modify the table of built-in modules if
Python hasn't been initialized. For Python embedded in LLDB, that means
this happen exactly once, before the first call to `Py_Initialize`,
which becomes a NO-OP after. However, when lldb is imported in an
existing Python interpreter, Python will have already been initialized,
but by definition, the lldb module will already have been loaded, so
it's safe to skip adding it (again).
This fixes#70453.
[1] https://docs.python.org/3.12/c-api/import.html#c.PyImport_AppendInittab
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