go.mod: use go.etcd.io/etcd/v3 versioning

This change makes the etcd package compatible with the existing Go
ecosystem for module versioning.

Used this tool to update package imports:
  https://github.com/KSubedi/gomove

pkg/flags/unique_urls.go

@@ -21,7 +21,7 @@ import (
 	"sort"
 	"strings"
 
-	"go.etcd.io/etcd/pkg/types"
+	"go.etcd.io/etcd/v3/pkg/types"
 )
 
 // UniqueURLs contains unique URLs

pkg/flags/urls.go

@@ -20,7 +20,7 @@ import (
 	"net/url"
 	"strings"
 
-	"go.etcd.io/etcd/pkg/types"
+	"go.etcd.io/etcd/v3/pkg/types"
 )
 
 // URLsValue wraps "types.URLs".