Create gopkgs directory for golang libs

- Created a gopkgs directory and registered it with default.nix's readTree
- Moved monzo_ynab/utils -> gopkgs
- Consumed utils.go in main.go
- Renamed monzo_ynab -> job

gopkgs/utils/default.nix

@@ -0,0 +1,11 @@
+{
+  depot ? import <depot> {},
+  ...
+}:
+
+depot.buildGo.package {
+  name = "utils";
+  srcs = [
+    ./utils.go
+  ];
+}