Starting v3 release for plugin developers.

[doc]: https://github.com/golang/go/wiki/Modules#releasing-modules-v2-or-higher

Per this Go modules [doc] a repo or branch that's
already tagged v2 or higher should increment the major
version (e.g. go to v3) when releasing their first Go
module-based packages.

At the moment, the kustomize repo has these top level
packages in the sigs.k8s.io/kustomize module:

 - `cmd` - holds main program for kustomize

	 Conceivably someone can depend on this
	 package for integration tests.

 - `internal` - intentionally unreleased subpackages

 - `k8sdeps` - an adapter wrapping k8s dependencies

	 This exists only for use in pre-Go-modules kustomize-into-kubectl
	 integration and won't live much longer (as everything involved is
	 switching to Go modules).

 - `pkg` - kustomize packages for export

	 This should shrink in later versions, since
	 the surface area is too large, containing
	 sub-packages that should be in 'internal'.

 - `plugin` - holds main programs for plugins

This PR changes the top level go.mod file from

```
module sigs.k8s.io/kustomize
```

to

```
module sigs.k8s.io/kustomize/v3
```

and adjusts all import statements to
reflect the change.

pkg/patch/strategicmerge.go

@@ -16,7 +16,7 @@ limitations under the License.
 
 package patch
 
-import "sigs.k8s.io/kustomize/pkg/types"
+import "sigs.k8s.io/kustomize/v3/pkg/types"
 
 // Append appends a slice of patch paths to a PatchStrategicMerge slice
 func Append(patches []types.PatchStrategicMerge, paths ...string) []types.PatchStrategicMerge {

pkg/patch/transformer/factory.go

@@ -19,11 +19,11 @@ package transformer
 import (
 	"fmt"
 
-	"sigs.k8s.io/kustomize/pkg/gvk"
-	"sigs.k8s.io/kustomize/pkg/ifc"
-	"sigs.k8s.io/kustomize/pkg/resid"
-	"sigs.k8s.io/kustomize/pkg/transformers"
-	"sigs.k8s.io/kustomize/pkg/types"
+	"sigs.k8s.io/kustomize/v3/pkg/gvk"
+	"sigs.k8s.io/kustomize/v3/pkg/ifc"
+	"sigs.k8s.io/kustomize/v3/pkg/resid"
+	"sigs.k8s.io/kustomize/v3/pkg/transformers"
+	"sigs.k8s.io/kustomize/v3/pkg/types"
 )
 
 // PatchJson6902Factory makes PatchJson6902 transformers

pkg/patch/transformer/factory_test.go

@@ -22,11 +22,11 @@ import (
 	"testing"
 
 	"gopkg.in/yaml.v2"
-	"sigs.k8s.io/kustomize/internal/loadertest"
-	"sigs.k8s.io/kustomize/k8sdeps/kunstruct"
-	"sigs.k8s.io/kustomize/pkg/resmaptest"
-	"sigs.k8s.io/kustomize/pkg/resource"
-	"sigs.k8s.io/kustomize/pkg/types"
+	"sigs.k8s.io/kustomize/v3/internal/loadertest"
+	"sigs.k8s.io/kustomize/v3/k8sdeps/kunstruct"
+	"sigs.k8s.io/kustomize/v3/pkg/resmaptest"
+	"sigs.k8s.io/kustomize/v3/pkg/resource"
+	"sigs.k8s.io/kustomize/v3/pkg/types"
 )
 
 var rf = resource.NewFactory(

pkg/patch/transformer/patchjson6902json.go

@@ -21,10 +21,10 @@ import (
 
 	jsonpatch "github.com/evanphx/json-patch"
 	"github.com/pkg/errors"
-	"sigs.k8s.io/kustomize/pkg/resid"
-	"sigs.k8s.io/kustomize/pkg/resmap"
-	"sigs.k8s.io/kustomize/pkg/resource"
-	"sigs.k8s.io/kustomize/pkg/transformers"
+	"sigs.k8s.io/kustomize/v3/pkg/resid"
+	"sigs.k8s.io/kustomize/v3/pkg/resmap"
+	"sigs.k8s.io/kustomize/v3/pkg/resource"
+	"sigs.k8s.io/kustomize/v3/pkg/transformers"
 	"sigs.k8s.io/yaml"
 )
 

pkg/patch/transformer/patchjson6902json_test.go

@@ -21,11 +21,11 @@ import (
 	"strings"
 	"testing"
 
-	"sigs.k8s.io/kustomize/k8sdeps/kunstruct"
-	"sigs.k8s.io/kustomize/pkg/gvk"
-	"sigs.k8s.io/kustomize/pkg/resid"
-	"sigs.k8s.io/kustomize/pkg/resmaptest"
-	"sigs.k8s.io/kustomize/pkg/resource"
+	"sigs.k8s.io/kustomize/v3/k8sdeps/kunstruct"
+	"sigs.k8s.io/kustomize/v3/pkg/gvk"
+	"sigs.k8s.io/kustomize/v3/pkg/resid"
+	"sigs.k8s.io/kustomize/v3/pkg/resmaptest"
+	"sigs.k8s.io/kustomize/v3/pkg/resource"
 )
 
 var deploy = gvk.Gvk{Group: "apps", Version: "v1", Kind: "Deployment"}