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)

overrides/buildGo/example/main.go

@@ -1,25 +0,0 @@
-// 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)
-}