Fix setup bug handling missing dependencies in unattended modes

[Jon TURNEY]

Here's a patch to fix a rather obscure setup bug, which I believe is probably
behind the bug report [1]

Test case:
1/ Run setup -M
2/ In the package chooser, uninstall a package which is a dependency of an
installed package
3/ Click next

[1] http://cygwin.com/ml/cygwin/2010-12/msg00178.html
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