invoice: calculate GTU codes for invoice, implement some tests
Also drive-by fix two proto issues:
- rename gtu_codes to gtu_code (following convention)
- move denormalized Item.due_date field past denormalized comment.
Change-Id: Ibfe0a21aadc0a5d4e2f784b182e530b9603aae62
diff --git a/bgpwtf/invoice/proto/BUILD.bazel b/bgpwtf/invoice/proto/BUILD.bazel
index 51f85fe..2eeae64 100644
--- a/bgpwtf/invoice/proto/BUILD.bazel
+++ b/bgpwtf/invoice/proto/BUILD.bazel
@@ -1,3 +1,4 @@
+load("@rules_proto//proto:defs.bzl", "proto_library")
load("@io_bazel_rules_go//go:def.bzl", "go_library")
load("@io_bazel_rules_go//proto:def.bzl", "go_proto_library")