[issue31415] Add -X option to show import time

[INADA Naoki]

INADA Naoki added the comment:

> If this proposition be accepted I would want to see not only cumulated times, but also pure times, without time of nested imports.

It can be calculated by script.
I don't want to add more complexity to import.c.

> I guess this feature doesn't work correctly with threading.

Yes, it should be documented.
And that's reason why I don't want to support time without nested import.
Stack management is difficult than inc/dec integer in multi threading case.

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<https://bugs.python.org/issue31415>
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