Add a make target: imports

We will use it to sort imports.

I tried openshift-goimports [1] and it did not work out
for us because

* It does not work with comments [2].
* We do have a comment to make golang linter happy [3].

[1]. https://github.com/openshift-eng/openshift-goimports

[2]. openshift-eng/openshift-goimports#16

[3]. https://github.com/openshift/cluster-version-operator/blob/ac127d3a5d45f60eb54e5c5acc3711a284728499/pkg/cvo/updatepayload_test.go#L14-L15

Author: hongkailiu

Makefile

@@ -34,14 +34,20 @@ format:
 	go fmt ./...
 .PHONY: format
 
+imports:
+	which goimports || go install -mod=mod golang.org/x/tools/cmd/goimports@latest
+	go list ./... | sed "s|^github.com/openshift/cluster-version-operator/||" | xargs goimports -w -local "k8s.io,github.com/openshift"
+	go list ./... | sed "s|^github.com/openshift/cluster-version-operator/||" | xargs goimports -w -local "github.com/openshift/cluster-version-operator"
+.PHONY: imports
+
 verify: verify-yaml verify-update
 .PHONY: verify
 
 verify-yaml:
 	hack/verify-yaml.sh
 .PHONY: verify-yaml
 
-verify-update: update
+verify-update: imports update
 	git diff --exit-code HEAD
 .PHONY: verify-update