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/generateBuiltins.sh

@@ -3,7 +3,7 @@
 # Generate the Go code for the generator and
 # transformer factory functions in
 #
-#   sigs.k8s.io/kustomize/plugin/builtin
+#   sigs.k8s.io/kustomize/v3/plugin/builtin
 #
 # from the raw plugin directories found _below_
 # that directory.
@@ -32,9 +32,9 @@ echo Generating linkable plugins...
 pushd $dir >& /dev/null
 
 GOPATH=$myGoPath go generate \
-    sigs.k8s.io/kustomize/plugin/builtin/...
+    sigs.k8s.io/kustomize/v3/plugin/builtin/...
 GOPATH=$myGoPath go fmt \
-    sigs.k8s.io/kustomize/plugin/builtin
+    sigs.k8s.io/kustomize/v3/plugin/builtin
 
 popd >& /dev/null