chore: Significantly restructure folder layout

This moves the various projects from "type-based" folders (such as
"services" or "tools") into more appropriate semantic folders (such as
"nix", "ops" or "web").

Deprecated projects (nixcon-demo & gotest) which only existed for
testing/demonstration purposes have been removed.

(Note: *all* builds are broken with this commit)

nix/buildGo/example/main.go

@@ -0,0 +1,25 @@
+// Copyright 2019 Google LLC.
+// SPDX-License-Identifier: Apache-2.0
+//
+// Package main provides a tiny example program for the Bazel-style
+// Nix build system for Go.
+
+package main
+
+import (
+	"example"
+	"exampleproto"
+	"fmt"
+)
+
+var Flag string = "unsuccessfully"
+
+func main() {
+	thing := exampleproto.Thing{
+		Id:          example.UUID(),
+		KindOfThing: "test thing",
+	}
+
+	fmt.Printf("The thing is a %s with ID %q\n", thing.Id, thing.KindOfThing)
+	fmt.Printf("The flag has been %s set\n", Flag)
+}