cmd/protoc-gen-go: group enum map vars in blocks

Since the enum maps are here to stay, group the declarations together
in a var block for better readability in godoc.

Change-Id: I9a313266539e9a60781f98b80a5293379f82607b
Reviewed-on: https://go-review.googlesource.com/c/protobuf/+/189077
Reviewed-by: Damien Neil <dneil@google.com>
diff --git a/encoding/testprotos/pb3/test.pb.go b/encoding/testprotos/pb3/test.pb.go
index 1b0958f..416faf1 100644
--- a/encoding/testprotos/pb3/test.pb.go
+++ b/encoding/testprotos/pb3/test.pb.go
@@ -27,19 +27,21 @@
 	Enum_TEN  Enum = 10
 )
 
-var Enum_name = map[int32]string{
-	0:  "ZERO",
-	1:  "ONE",
-	2:  "TWO",
-	10: "TEN",
-}
-
-var Enum_value = map[string]int32{
-	"ZERO": 0,
-	"ONE":  1,
-	"TWO":  2,
-	"TEN":  10,
-}
+// Enum value maps for Enum.
+var (
+	Enum_name = map[int32]string{
+		0:  "ZERO",
+		1:  "ONE",
+		2:  "TWO",
+		10: "TEN",
+	}
+	Enum_value = map[string]int32{
+		"ZERO": 0,
+		"ONE":  1,
+		"TWO":  2,
+		"TEN":  10,
+	}
+)
 
 func (x Enum) Enum() *Enum {
 	p := new(Enum)
@@ -77,19 +79,21 @@
 	Enums_DIEZ Enums_NestedEnum = 10
 )
 
-var Enums_NestedEnum_name = map[int32]string{
-	0:  "CERO",
-	1:  "UNO",
-	2:  "DOS",
-	10: "DIEZ",
-}
-
-var Enums_NestedEnum_value = map[string]int32{
-	"CERO": 0,
-	"UNO":  1,
-	"DOS":  2,
-	"DIEZ": 10,
-}
+// Enum value maps for Enums_NestedEnum.
+var (
+	Enums_NestedEnum_name = map[int32]string{
+		0:  "CERO",
+		1:  "UNO",
+		2:  "DOS",
+		10: "DIEZ",
+	}
+	Enums_NestedEnum_value = map[string]int32{
+		"CERO": 0,
+		"UNO":  1,
+		"DOS":  2,
+		"DIEZ": 10,
+	}
+)
 
 func (x Enums_NestedEnum) Enum() *Enums_NestedEnum {
 	p := new(Enums_NestedEnum)