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/transformers/config/factory.go

@@ -19,7 +19,7 @@ package config
 import (
 	"log"
 
-	"sigs.k8s.io/kustomize/pkg/ifc"
+	"sigs.k8s.io/kustomize/v3/pkg/ifc"
 	"sigs.k8s.io/yaml"
 )
 

pkg/transformers/config/factory_test.go

@@ -7,8 +7,8 @@ import (
 	"reflect"
 	"testing"
 
-	"sigs.k8s.io/kustomize/internal/loadertest"
-	"sigs.k8s.io/kustomize/pkg/gvk"
+	"sigs.k8s.io/kustomize/v3/internal/loadertest"
+	"sigs.k8s.io/kustomize/v3/pkg/gvk"
 )
 
 func TestMakeDefaultConfig(t *testing.T) {

pkg/transformers/config/factorycrd.go

@@ -23,8 +23,8 @@ import (
 	"github.com/go-openapi/spec"
 	"github.com/pkg/errors"
 	"k8s.io/kube-openapi/pkg/common"
-	"sigs.k8s.io/kustomize/pkg/gvk"
-	"sigs.k8s.io/kustomize/pkg/ifc"
+	"sigs.k8s.io/kustomize/v3/pkg/gvk"
+	"sigs.k8s.io/kustomize/v3/pkg/ifc"
 	"sigs.k8s.io/yaml"
 )
 

pkg/transformers/config/factorycrd_test.go

@@ -20,9 +20,9 @@ import (
 	"reflect"
 	"testing"
 
-	"sigs.k8s.io/kustomize/internal/loadertest"
-	"sigs.k8s.io/kustomize/pkg/gvk"
-	"sigs.k8s.io/kustomize/pkg/ifc"
+	"sigs.k8s.io/kustomize/v3/internal/loadertest"
+	"sigs.k8s.io/kustomize/v3/pkg/gvk"
+	"sigs.k8s.io/kustomize/v3/pkg/ifc"
 )
 
 // This defines two CRD's:  Bee and MyKind.

pkg/transformers/config/fieldspec.go

@@ -20,7 +20,7 @@ import (
 	"fmt"
 	"strings"
 
-	"sigs.k8s.io/kustomize/pkg/gvk"
+	"sigs.k8s.io/kustomize/v3/pkg/gvk"
 )
 
 // FieldSpec completely specifies a kustomizable field in

pkg/transformers/config/fieldspec_test.go

@@ -22,7 +22,7 @@ import (
 	"strings"
 	"testing"
 
-	"sigs.k8s.io/kustomize/pkg/gvk"
+	"sigs.k8s.io/kustomize/v3/pkg/gvk"
 )
 
 func TestPathSlice(t *testing.T) {

pkg/transformers/config/namebackreferences.go

@@ -19,7 +19,7 @@ package config
 import (
 	"strings"
 
-	"sigs.k8s.io/kustomize/pkg/gvk"
+	"sigs.k8s.io/kustomize/v3/pkg/gvk"
 )
 
 // NameBackReferences is an association between a gvk.GVK and a list

pkg/transformers/config/namebackreferences_test.go

@@ -18,7 +18,7 @@ package config
 
 import (
 	"reflect"
-	"sigs.k8s.io/kustomize/pkg/gvk"
+	"sigs.k8s.io/kustomize/v3/pkg/gvk"
 	"testing"
 )
 

pkg/transformers/config/transformerconfig.go

@@ -22,7 +22,7 @@ import (
 	"log"
 	"sort"
 
-	"sigs.k8s.io/kustomize/pkg/transformers/config/defaultconfig"
+	"sigs.k8s.io/kustomize/v3/pkg/transformers/config/defaultconfig"
 )
 
 // TransformerConfig holds the data needed to perform transformations.

pkg/transformers/config/transformerconfig_test.go

@@ -21,7 +21,7 @@ import (
 
 	"reflect"
 
-	"sigs.k8s.io/kustomize/pkg/gvk"
+	"sigs.k8s.io/kustomize/v3/pkg/gvk"
 )
 
 func TestAddNamereferenceFieldSpec(t *testing.T) {