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/doc.go

@@ -92,7 +92,7 @@ The kind is the CamelCase name of the plugin.
 The source for a builtin plugin must be at:
 
   repo=$GOPATH/src/sigs.k8s.io/kustomize
-  ${repo}/plugin/builtin/LOWERCASE(${kind})/${kind}
+  ${repo}/v3/plugin/builtin/LOWERCASE(${kind})/${kind}
 
 k8s wants 'kind' values to follow CamelCase, while
 Go style doesn't like but does allow such names.
@@ -156,7 +156,7 @@ code, hence this program.
 TO GENERATE CODE
 
   repo=$GOPATH/src/sigs.k8s.io/kustomize
-  cd $repo/plugin/builtin
+  cd $repo/v3/plugin/builtin
   go generate ./...
 
 This creates
@@ -168,7 +168,7 @@ etc.
 Generated plugins are used in kustomize via
 
   package whatever
-  import "sigs.k8s.io/kustomize/plugin/builtin
+  import "sigs.k8s.io/kustomize/v3/plugin/builtin
   ...
   g := builtin.NewSecretGenerator()
   g.Config(l, rf, k)