Merge pull request #57259 from ericchiang/client-go-no-cache-import

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>.

client-go: remove import of github.com/gregjones/httpcache

Moves NewCacheRoundTripper from `k8s.io/client-go/transport` to its own package. This prevents Kubernetes clients from requiring its dependencies.

This change removes the following transitive imports from `k8s.io/client-go/kubernetes`

```
github.com/google/btree
github.com/gregjones/httpcache
github.com/gregjones/httpcache/diskcache
github.com/peterbourgon/diskv
```

```release-note
NONE
```

Kubernetes-commit: 4a77bd53e88a719c612086df69a52899bbdd85af

vendor/github.com/gregjones/httpcache/.travis.yml

@@ -1,18 +0,0 @@
-sudo: false
-language: go
-go:
-  - 1.6.x
-  - 1.7.x
-  - 1.8.x
-  - master
-matrix:
-  allow_failures:
-    - go: master
-  fast_finish: true
-install:
-  - # Do nothing. This is needed to prevent default install action "go get -t -v ./..." from happening here (we want it to happen inside script step).
-script:
-  - go get -t -v ./...
-  - diff -u <(echo -n) <(gofmt -d .)
-  - go tool vet .
-  - go test -v -race ./...