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/storage/storage.go

@@ -0,0 +1,51 @@
+// 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
+}