Configure goimports linter to group internal packages (nginx#123)

* Configure goimports linter to group internal packages

* Make imports statement consistent

Author: miledxz

.golangci.yml

@@ -1,4 +1,6 @@
 linters-settings:
+  goimports:
+    local-prefixes: github.com/nginxinc/nginx-kubernetes-gateway
   misspell:
     locale: US
   revive:

cmd/gateway/setup_test.go

@@ -3,11 +3,11 @@ package main_test
 import (
 	"errors"
 
-	. "github.com/nginxinc/nginx-kubernetes-gateway/cmd/gateway"
-	flag "github.com/spf13/pflag"
-
 	. "github.com/onsi/ginkgo"
 	. "github.com/onsi/gomega"
+	flag "github.com/spf13/pflag"
+
+	. "github.com/nginxinc/nginx-kubernetes-gateway/cmd/gateway"
 )
 
 var domain string