goimports (#16640)

* format using `goimports -local github.com/mattermost/mattermost-server/v5 -w`

* added goimports lint check to .golangci.yml

* format using `goimports -local github.com/mattermost/mattermost-server/v5 -w` for a corner case

* make app-layers, *-mocks and store-layers for ci check

Co-authored-by: Mahmudul Haque <mahmudulhaque@protonmail.com>
Co-authored-by: Mattermod <mattermod@users.noreply.github.com>

plugin/stringifier_test.go

@@ -4,9 +4,10 @@
 package plugin
 
 import (
+	"testing"
+
 	"github.com/pkg/errors"
 	"github.com/stretchr/testify/assert"
-	"testing"
 )
 
 func TestStringify(t *testing.T) {