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/stringprefixer/StringPrefixer.go

@@ -5,11 +5,11 @@ package main
 
 import (
 	"github.com/pkg/errors"
-	"sigs.k8s.io/kustomize/pkg/ifc"
-	"sigs.k8s.io/kustomize/pkg/resmap"
-	"sigs.k8s.io/kustomize/pkg/transformers"
-	"sigs.k8s.io/kustomize/pkg/transformers/config"
-	"sigs.k8s.io/kustomize/plugin/builtin"
+	"sigs.k8s.io/kustomize/v3/pkg/ifc"
+	"sigs.k8s.io/kustomize/v3/pkg/resmap"
+	"sigs.k8s.io/kustomize/v3/pkg/transformers"
+	"sigs.k8s.io/kustomize/v3/pkg/transformers/config"
+	"sigs.k8s.io/kustomize/v3/plugin/builtin"
 	"sigs.k8s.io/yaml"
 )