[WIP] 🏃 External k8s.io References Checker

[DirectXMan12]

This introduces a tool for checking all our external references to
Kubernetes in our public API surface, so that we can make sure we don't
accidentally break people in an update due to changing Kubernetes API
surface.

[k8s-ci-robot]

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[DirectXMan12]

cc @droot @mengqiy @pwittrock part one of the tooling

[DirectXMan12]

(may need a bit of cleanup before merge, but wanted to get it up for review)

[DirectXMan12]

Note to future self: it looks like there's a prebuilt version of something similar to this hiding in https://godoc.org/golang.org/x/exp/apidiff. I can probably make use of it to simplify this code.

[mengqiy]

@DirectXMan12 Just be aware of x/exp/apidiff is still experimental and not stable. That mean each time we upgrade to a newer go version, it may break.

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[DirectXMan12]

closing, but I'll bring this back at some point