go/neo/proto: protogen: Factor-out code to lookup object for <pkg>.<name> into common function

Besides proto.customCodec we will need to lookup more objects.

go/neo/proto/protogen.go

@@ -253,10 +253,7 @@ func main() {
 	protoPkg = loadPkg("lab.nexedi.com/kirr/neo/go/neo/proto", "proto.go")
 
 	// extract neo.customCodec
-	cc := protoPkg.Scope().Lookup("customCodec")
-	if cc == nil {
-		log.Fatal("cannot find `customCodec`")
-	}
+	cc := xlookup(protoPkg, "customCodec")
 	var ok bool
 	neo_customCodec, ok = cc.Type().Underlying().(*types.Interface)
 	if !ok {
@@ -1252,6 +1249,16 @@ func generateCodecCode(typespec *ast.TypeSpec, codegen CodeGenerator) string {
 	return codegen.generatedCode()
 }
 
+// xlookup looks up <pkg>.<name> object.
+// It is fatal error if the object cannot be found.
+func xlookup(pkg *types.Package, name string) types.Object {
+	obj := pkg.Scope().Lookup(name)
+	if obj == nil {
+		log.Fatalf("cannot find `%s.%s`", pkg.Name(), name)
+	}
+	return obj
+}
+
 // isByte returns whether typ represents byte.
 func isByte(typ types.Type) bool {
 	t, ok := typ.(*types.Basic)