Automatic merge from submit-queue. If you want to cherry-pick this change to another branch, please follow the instructions <a href="https://github.com/kubernetes/community/blob/master/contributors/devel/cherry-picks.md">here</a>.
code-gen: reusable main.go logic, preparation for kube-gen
Split main.go files into plumbing and calls to NewDefaults, AddFlags and Validate. This will allow us to create kube-gen without duplicating much code, at least no generator logic (with the exception of a little loop in client-gen adding the group path to the input dirs).
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