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.

k8sdeps/transformer/factory.go

@@ -5,9 +5,9 @@
 package transformer
 
 import (
-	"sigs.k8s.io/kustomize/k8sdeps/transformer/patch"
-	"sigs.k8s.io/kustomize/pkg/resource"
-	"sigs.k8s.io/kustomize/pkg/transformers"
+	"sigs.k8s.io/kustomize/v3/k8sdeps/transformer/patch"
+	"sigs.k8s.io/kustomize/v3/pkg/resource"
+	"sigs.k8s.io/kustomize/v3/pkg/transformers"
 )
 
 // FactoryImpl makes patch transformer and name hash transformer

k8sdeps/transformer/patch/conflictdetector.go

@@ -10,7 +10,7 @@ import (
 	"k8s.io/apimachinery/pkg/runtime"
 	"k8s.io/apimachinery/pkg/util/mergepatch"
 	"k8s.io/apimachinery/pkg/util/strategicpatch"
-	"sigs.k8s.io/kustomize/pkg/resource"
+	"sigs.k8s.io/kustomize/v3/pkg/resource"
 )
 
 type conflictDetector interface {

k8sdeps/transformer/patch/transformer.go

@@ -12,11 +12,11 @@ import (
 	"k8s.io/apimachinery/pkg/runtime/schema"
 	"k8s.io/apimachinery/pkg/util/strategicpatch"
 	"k8s.io/client-go/kubernetes/scheme"
-	"sigs.k8s.io/kustomize/pkg/gvk"
-	"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/gvk"
+	"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"
 )
 
 // transformer applies strategic merge patches.

k8sdeps/transformer/patch/transformer_test.go

@@ -8,9 +8,9 @@ import (
 	"strings"
 	"testing"
 
-	"sigs.k8s.io/kustomize/k8sdeps/kunstruct"
-	"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/resmaptest"
+	"sigs.k8s.io/kustomize/v3/pkg/resource"
 )
 
 var rf = resource.NewFactory(