cmd/protoc-gen-go: remove json struct tags from unexported fields

This commit does not change behavior because the encoding/json
package ignores unexported fields. To encode a Protobuf message
into JSON, users should reach for protobuf/encoding/protojson.

With this change, the generated code (.pb.go files) passes go vet.
I also made the integration test run go vet so that we do not regress.

For golang/protobuf#1657

Fixes golang/protobuf#1658

Change-Id: I0a677d09e4517b2649bfcfe76a49194c7ccbb823
Reviewed-on: https://go-review.googlesource.com/c/protobuf/+/641036
Reviewed-by: Damien Neil <[email protected]>
Reviewed-by: Chressie Himpel <[email protected]>
LUCI-TryBot-Result: Go LUCI <[email protected]>

Author: stapelberg

cmd/protoc-gen-go/internal_gengo/opaque.go

@@ -93,7 +93,9 @@ func opaqueGenMessageField(g *protogen.GeneratedFile, f *fileInfo, message *mess
 	}
 	tags := structTags{
 		{"protobuf", protobufTagValue},
-		{"json", jsonTagValue},
+	}
+	if !message.isOpaque() {
+		tags = append(tags, structTags{{"json", jsonTagValue}}...)
 	}
 	if field.Desc.IsMap() {
 		keyTagValue := fieldProtobufTagValue(field.Message.Fields[0])