[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.
* use one place to build plugin file names,
* use one loader instance,
* test for plugin enabled flag in just one place to
avoid errors and reduce if statements,
* don't return private objects,
* factor goplugin loading to a method,
* fix a related test that was commented out.
