Use underlying `NewKubernetesConfig` directly from compose-on-kubernetes.

The comment on `github.com/docker/cli/kubernetes.NewKubernetesConfig` said:

    // Deprecated: Use github.com/docker/compose-on-kubernetes/api.NewKubernetesConfig instead

By making this switch in `github.com/docker/cli/context/kubernetes/load.go` we
break a vendoring chain:

`github.com/docker/cli/cli/command`
→ `vendor/github.com/docker/cli/cli/context/kubernetes/load.go`
  → `vendor/github.com/docker/cli/kubernetes`
     → `github.com/docker/compose-on-kubernetes/api/compose/...`

This means that projects which just want `github.com/docker/cli/cli/command`
(which is itself pulled in transitively by
`github.com/docker/cli/cli-plugins/plugin`) which do not themselves need the
compose-on-kubernetes API avoid a huge pile of transitive dependencies.

On one of my private projects the diff on the vendor dir is:

    280 files changed, 21 insertions(+), 211346 deletions(-)

and includes dropping:

* `github.com/docker/compose-on-kubernetes/api/compose/{clone,impersonation}`
* `github.com/docker/compose-on-kubernetes/api/compose/{v1alpha3,v1beta1,v1beta2,v1beta3}`
* `github.com/google/btree`
* `github.com/googleapis/gnostic`
* `github.com/gregjones/httpcache`
* `github.com/peterbourgon/diskv`
* `k8s.io/api/*` (_lots_ of subpackages)
* `k8s.io/client-go/{discovery,kubernetes/scheme}`

and I've gone from:

    $ du -sh vendor/k8s.io/
    8.1M	vendor/k8s.io/

to:

    $ du -sh vendor/k8s.io/
    2.1M	vendor/k8s.io/

(overall I went from 36M → 29M of vendor dir for this particular project)

The change to `cli/command/system/version.go` is just for consistency and
allows us to drop the now unused alias.

Signed-off-by: Ian Campbell <ijc@docker.com>

cli/command/system/version.go

@@ -15,6 +15,7 @@ import (
 	"github.com/docker/cli/cli/version"
 	"github.com/docker/cli/kubernetes"
 	"github.com/docker/cli/templates"
+	kubeapi "github.com/docker/compose-on-kubernetes/api"
 	"github.com/docker/docker/api/types"
 	"github.com/pkg/errors"
 	"github.com/sirupsen/logrus"
@@ -243,7 +244,7 @@ func getKubernetesVersion(dockerCli command.Cli, kubeConfig string) *kubernetesV
 		err          error
 	)
 	if dockerCli.CurrentContext() == "" {
-		clientConfig = kubernetes.NewKubernetesConfig(kubeConfig)
+		clientConfig = kubeapi.NewKubernetesConfig(kubeConfig)
 	} else {
 		clientConfig, err = kubecontext.ConfigFromContext(dockerCli.CurrentContext(), dockerCli.ContextStore())
 	}

cli/context/kubernetes/load.go

@@ -3,7 +3,7 @@ package kubernetes
 import (
 	"github.com/docker/cli/cli/context"
 	"github.com/docker/cli/cli/context/store"
-	"github.com/docker/cli/kubernetes"
+	api "github.com/docker/compose-on-kubernetes/api"
 	"k8s.io/client-go/tools/clientcmd"
 	clientcmdapi "k8s.io/client-go/tools/clientcmd/api"
 )
@@ -91,5 +91,5 @@ func ConfigFromContext(name string, s store.Reader) (clientcmd.ClientConfig, err
 		return ep.KubernetesConfig(), nil
 	}
 	// context has no kubernetes endpoint
-	return kubernetes.NewKubernetesConfig(""), nil
+	return api.NewKubernetesConfig(""), nil
 }

kubernetes/config.go

@@ -1,8 +0,0 @@
-package kubernetes
-
-import api "github.com/docker/compose-on-kubernetes/api"
-
-// NewKubernetesConfig resolves the path to the desired Kubernetes configuration file based on
-// the KUBECONFIG environment variable and command line flags.
-// Deprecated: Use github.com/docker/compose-on-kubernetes/api.NewKubernetesConfig instead
-var NewKubernetesConfig = api.NewKubernetesConfig