gomod: run go mod tidy for all modules

Also add the `make tidy-module` copied from `lnd`.

scripts/tidy_modules.sh

@@ -0,0 +1,17 @@
+#!/bin/bash
+
+SUBMODULES=$(find . -mindepth 2 -name "go.mod" | cut -d'/' -f2)
+
+
+# Run 'go mod tidy' for root.
+go mod tidy
+
+# Run 'go mod tidy' for each module.
+for submodule in $SUBMODULES
+do
+  pushd $submodule
+
+  go mod tidy
+
+  popd
+done