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/loader/fileloader.go

@@ -9,9 +9,9 @@ import (
 	"path/filepath"
 	"strings"
 
-	"sigs.k8s.io/kustomize/pkg/fs"
-	"sigs.k8s.io/kustomize/pkg/git"
-	"sigs.k8s.io/kustomize/pkg/ifc"
+	"sigs.k8s.io/kustomize/v3/pkg/fs"
+	"sigs.k8s.io/kustomize/v3/pkg/git"
+	"sigs.k8s.io/kustomize/v3/pkg/ifc"
 )
 
 // fileLoader is a kustomization's interface to files.

pkg/loader/fileloader_test.go

@@ -25,11 +25,11 @@ import (
 	"strings"
 	"testing"
 
-	"sigs.k8s.io/kustomize/pkg/fs"
-	"sigs.k8s.io/kustomize/pkg/git"
-	"sigs.k8s.io/kustomize/pkg/ifc"
-	"sigs.k8s.io/kustomize/pkg/pgmconfig"
-	"sigs.k8s.io/kustomize/pkg/validators"
+	"sigs.k8s.io/kustomize/v3/pkg/fs"
+	"sigs.k8s.io/kustomize/v3/pkg/git"
+	"sigs.k8s.io/kustomize/v3/pkg/ifc"
+	"sigs.k8s.io/kustomize/v3/pkg/pgmconfig"
+	"sigs.k8s.io/kustomize/v3/pkg/validators"
 )
 
 type testData struct {

pkg/loader/kv.go

@@ -14,8 +14,8 @@ import (
 	"unicode/utf8"
 
 	"github.com/pkg/errors"
-	"sigs.k8s.io/kustomize/pkg/ifc"
-	"sigs.k8s.io/kustomize/pkg/types"
+	"sigs.k8s.io/kustomize/v3/pkg/ifc"
+	"sigs.k8s.io/kustomize/v3/pkg/types"
 )
 
 var utf8bom = []byte{0xEF, 0xBB, 0xBF}

pkg/loader/kv_test.go

@@ -7,9 +7,9 @@ import (
 	"reflect"
 	"testing"
 
-	"sigs.k8s.io/kustomize/pkg/fs"
-	"sigs.k8s.io/kustomize/pkg/types"
-	"sigs.k8s.io/kustomize/pkg/validators"
+	"sigs.k8s.io/kustomize/v3/pkg/fs"
+	"sigs.k8s.io/kustomize/v3/pkg/types"
+	"sigs.k8s.io/kustomize/v3/pkg/validators"
 )
 
 func TestKeyValuesFromLines(t *testing.T) {

pkg/loader/loader.go

@@ -18,9 +18,9 @@ limitations under the License.
 package loader
 
 import (
-	"sigs.k8s.io/kustomize/pkg/fs"
-	"sigs.k8s.io/kustomize/pkg/git"
-	"sigs.k8s.io/kustomize/pkg/ifc"
+	"sigs.k8s.io/kustomize/v3/pkg/fs"
+	"sigs.k8s.io/kustomize/v3/pkg/git"
+	"sigs.k8s.io/kustomize/v3/pkg/ifc"
 )
 
 // NewLoader returns a Loader pointed at the given target.

pkg/loader/loadrestrictions.go

@@ -20,7 +20,7 @@ import (
 	"fmt"
 
 	"github.com/spf13/pflag"
-	"sigs.k8s.io/kustomize/pkg/fs"
+	"sigs.k8s.io/kustomize/v3/pkg/fs"
 )
 
 //go:generate stringer -type=loadRestrictions

pkg/loader/loadrestrictions_test.go

@@ -20,7 +20,7 @@ import (
 	"strings"
 	"testing"
 
-	"sigs.k8s.io/kustomize/pkg/fs"
+	"sigs.k8s.io/kustomize/v3/pkg/fs"
 )
 
 func TestRestrictionNone(t *testing.T) {