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/manifest/manifest.go

@@ -0,0 +1,141 @@
+// 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 image implements logic for creating the image metadata
+// (such as the image manifest and configuration).
+package manifest
+
+import (
+	"crypto/sha256"
+	"encoding/json"
+	"fmt"
+	"sort"
+)
+
+const (
+	// manifest constants
+	schemaVersion = 2
+
+	// media types
+	manifestType = "application/vnd.docker.distribution.manifest.v2+json"
+	layerType    = "application/vnd.docker.image.rootfs.diff.tar.gzip"
+	configType   = "application/vnd.docker.container.image.v1+json"
+
+	// image config constants
+	os     = "linux"
+	fsType = "layers"
+)
+
+type Entry struct {
+	MediaType string `json:"mediaType,omitempty"`
+	Size      int64  `json:"size"`
+	Digest    string `json:"digest"`
+
+	// These fields are internal to Nixery and not part of the
+	// serialised entry.
+	MergeRating uint64 `json:"-"`
+	TarHash     string `json:",omitempty"`
+}
+
+type manifest struct {
+	SchemaVersion int     `json:"schemaVersion"`
+	MediaType     string  `json:"mediaType"`
+	Config        Entry   `json:"config"`
+	Layers        []Entry `json:"layers"`
+}
+
+type imageConfig struct {
+	Architecture string `json:"architecture"`
+	OS           string `json:"os"`
+
+	RootFS struct {
+		FSType  string   `json:"type"`
+		DiffIDs []string `json:"diff_ids"`
+	} `json:"rootfs"`
+
+	// sic! empty struct (rather than `null`) is required by the
+	// image metadata deserialiser in Kubernetes
+	Config struct{} `json:"config"`
+}
+
+// ConfigLayer represents the configuration layer to be included in
+// the manifest, containing its JSON-serialised content and SHA256
+// hash.
+type ConfigLayer struct {
+	Config []byte
+	SHA256 string
+}
+
+// imageConfig creates an image configuration with the values set to
+// the constant defaults.
+//
+// Outside of this module the image configuration is treated as an
+// opaque blob and it is thus returned as an already serialised byte
+// array and its SHA256-hash.
+func configLayer(arch string, hashes []string) ConfigLayer {
+	c := imageConfig{}
+	c.Architecture = arch
+	c.OS = os
+	c.RootFS.FSType = fsType
+	c.RootFS.DiffIDs = hashes
+
+	j, _ := json.Marshal(c)
+
+	return ConfigLayer{
+		Config: j,
+		SHA256: fmt.Sprintf("%x", sha256.Sum256(j)),
+	}
+}
+
+// Manifest creates an image manifest from the specified layer entries
+// and returns its JSON-serialised form as well as the configuration
+// layer.
+//
+// Callers do not need to set the media type for the layer entries.
+func Manifest(arch string, layers []Entry) (json.RawMessage, ConfigLayer) {
+	// Sort layers by their merge rating, from highest to lowest.
+	// This makes it likely for a contiguous chain of shared image
+	// layers to appear at the beginning of a layer.
+	//
+	// Due to moby/moby#38446 Docker considers the order of layers
+	// when deciding which layers to download again.
+	sort.Slice(layers, func(i, j int) bool {
+		return layers[i].MergeRating > layers[j].MergeRating
+	})
+
+	hashes := make([]string, len(layers))
+	for i, l := range layers {
+		hashes[i] = l.TarHash
+		l.MediaType = layerType
+		l.TarHash = ""
+		layers[i] = l
+	}
+
+	c := configLayer(arch, hashes)
+
+	m := manifest{
+		SchemaVersion: schemaVersion,
+		MediaType:     manifestType,
+		Config: Entry{
+			MediaType: configType,
+			Size:      int64(len(c.Config)),
+			Digest:    "sha256:" + c.SHA256,
+		},
+		Layers: layers,
+	}
+
+	j, _ := json.Marshal(m)
+
+	return json.RawMessage(j), c
+}