Generate vendor directories

[terrytangyuan]

Generated vendor directories via go mod vendor similar to what we've done for kubeflow/common (see discussion in kubeflow/common#28).

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This change is 

[merlintang]

/lgtm

[k8s-ci-robot]

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[merlintang]

/approve

[terrytangyuan]

/approve

[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is APPROVED

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[terrytangyuan]

/lgtm

[k8s-ci-robot]

@terrytangyuan: you cannot LGTM your own PR.

In response to this:

/lgtm

Instructions for interacting with me using PR comments are available here. If you have questions or suggestions related to my behavior, please file an issue against the kubernetes/test-infra repository.

[terrytangyuan]

Handing over to you for lgtm:
/assign @johnugeorge @richardsliu

[johnugeorge]

/lgtm