[IronPython] Bug in 2.0a1 SystemState.InitializeBuiltins

[Sanghyeon Seo]

I hope the bug is obvious once pointed out. InitializeBuiltins method
contains a code to replace built-in nt module with posix module when
Platform is PlatformID.Unix. But this is done *before*
IronPython.Modules is loaded! As a result, KeyNotFoundException is
raised.

Clearly, this won't happen on Windows.

-- 
Seo Sanghyeon