fix directives related to go generate in package spdx (google#730)

Author: josieang

pkg/spdx/gen.go

@@ -1,6 +1,7 @@
-//go:generate go run .
-//go:build exclude
-// +build exclude
+//go:build generate
+// +build generate
+
+//go:generate go run gen.go
 
 package main
 
@@ -37,7 +38,7 @@ func main() {
 		panic(err)
 	}
 
-	output := "package spdx\nvar IDs = map[string]bool{\n"
+	output := "// Code generated by gen.go. DO NOT EDIT.\n package spdx\nvar IDs = map[string]bool{\n"
 	for _, license := range licenseList.Licenses {
 		output += fmt.Sprintf("%q: true,\n", strings.ToLower(license.SPDXID))
 	}