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.

plugin/someteam.example.com/v1/secretsfromdatabase/SecretsFromDatabase_test.go

@@ -6,12 +6,12 @@ package main_test
 import (
 	"testing"
 
-	"sigs.k8s.io/kustomize/pkg/kusttest"
-	"sigs.k8s.io/kustomize/plugin"
+	"sigs.k8s.io/kustomize/v3/pkg/kusttest"
+	"sigs.k8s.io/kustomize/v3/pkg/plugins"
 )
 
 func TestSecretsFromDatabasePlugin(t *testing.T) {
-	tc := plugin.NewEnvForTest(t).Set()
+	tc := plugins.NewEnvForTest(t).Set()
 	defer tc.Reset()
 
 	tc.BuildGoPlugin(