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>

store/storetest/mocks/ObjectCache.go

@@ -4,7 +4,9 @@
 
 package mocks
 
-import mock "github.com/stretchr/testify/mock"
+import (
+	mock "github.com/stretchr/testify/mock"
+)
 
 // ObjectCache is an autogenerated mock type for the ObjectCache type
 type ObjectCache struct {