refactor: Reshuffle file structure for better code layout

This gets rid of the package called "server" and instead moves
everything into the project root, such that Go actually builds us a
binary called `nixery`.

This is the first step towards factoring out CLI-based functionality
for Nixery.

tools/nixery/server/storage/storage.go

@@ -1,51 +0,0 @@
-// Copyright 2019 Google LLC
-//
-// Licensed under the Apache License, Version 2.0 (the "License"); you may not
-// use this file except in compliance with the License. You may obtain a copy of
-// the License at
-//
-//     https://www.apache.org/licenses/LICENSE-2.0
-//
-// Unless required by applicable law or agreed to in writing, software
-// distributed under the License is distributed on an "AS IS" BASIS, WITHOUT
-// WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. See the
-// License for the specific language governing permissions and limitations under
-// the License.
-
-// Package storage implements an interface that can be implemented by
-// storage backends, such as Google Cloud Storage or the local
-// filesystem.
-package storage
-
-import (
-	"context"
-	"io"
-	"net/http"
-)
-
-type Persister = func(io.Writer) (string, int64, error)
-
-type Backend interface {
-	// Name returns the name of the storage backend, for use in
-	// log messages and such.
-	Name() string
-
-	// Persist provides a user-supplied function with a writer
-	// that stores data in the storage backend.
-	//
-	// It needs to return the SHA256 hash of the data written as
-	// well as the total number of bytes, as those are required
-	// for the image manifest.
-	Persist(context.Context, string, Persister) (string, int64, error)
-
-	// Fetch retrieves data from the storage backend.
-	Fetch(ctx context.Context, path string) (io.ReadCloser, error)
-
-	// Move renames a path inside the storage backend. This is
-	// used for staging uploads while calculating their hashes.
-	Move(ctx context.Context, old, new string) error
-
-	// Serve provides a handler function to serve HTTP requests
-	// for layers in the storage backend.
-	ServeLayer(digest string, r *http.Request, w http.ResponseWriter) error
-}