…-staging

Automatic merge from submit-queue

Cut the client repo, staging it in the main repo

Tracking issue: kubernetes#28559
ref: kubernetes#25978 (comment)

This PR implements the plan a few of us came up with last week for cutting client into its own repo:
1. creating "_staging" (name is tentative) directory in the main repo, using a script to copy the client and its dependencies to this directory
2. periodically publishing the contents of this staging client to k8s.io/client-go repo
3. converting k8s components in the main repo to use the staged client. They should import the staged client as if the client were vendored. (i.e., the import line should be `import "k8s.io/client-go/<pacakge name>`). This requirement is to ease step 4.
4. In the future, removing the staging area, and vendoring the real client-go repo.

The advantage of having the staging area is that we can continuously run integration/e2e tests with the latest client repo and the latest main repo, without waiting for the client repo to be vendored back into the main repo. This staging area will exist until our test matrix is vendoring both the client and the server.

In the above plan, the tricky part is step 3. This PR achieves it by creating a symlink under ./vendor, pointing to the staging area, so packages in the main repo can refer to the client repo as if it's vendored. To prevent the godep tool from messing up the staging area, we export the staged client to GOPATH in hack/godep-save.sh so godep will think the client packages are local and won't attempt to manage ./vendor/k8s.io/client-go.

This is a POC. We'll rearrange the directory layout of the client before merge.

@thockin @lavalamp @bgrant0607 @kubernetes/sig-api-machinery

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