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/config/pkgsource.go

@@ -0,0 +1,159 @@
+// 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 config
+
+import (
+	"crypto/sha1"
+	"encoding/json"
+	"fmt"
+	"os"
+	"regexp"
+	"strings"
+
+	log "github.com/sirupsen/logrus"
+)
+
+// PkgSource represents the source from which the Nix package set used
+// by Nixery is imported. Users configure the source by setting one of
+// the supported environment variables.
+type PkgSource interface {
+	// Convert the package source into the representation required
+	// for calling Nix.
+	Render(tag string) (string, string)
+
+	// Create a key by which builds for this source and iamge
+	// combination can be cached.
+	//
+	// The empty string means that this value is not cacheable due
+	// to the package source being a moving target (such as a
+	// channel).
+	CacheKey(pkgs []string, tag string) string
+}
+
+type GitSource struct {
+	repository string
+}
+
+// Regex to determine whether a git reference is a commit hash or
+// something else (branch/tag).
+//
+// Used to check whether a git reference is cacheable, and to pass the
+// correct git structure to Nix.
+//
+// Note: If a user creates a branch or tag with the name of a commit
+// and references it intentionally, this heuristic will fail.
+var commitRegex = regexp.MustCompile(`^[0-9a-f]{40}$`)
+
+func (g *GitSource) Render(tag string) (string, string) {
+	args := map[string]string{
+		"url": g.repository,
+	}
+
+	// The 'git' source requires a tag to be present. If the user
+	// has not specified one, it is assumed that the default
+	// 'master' branch should be used.
+	if tag == "latest" || tag == "" {
+		tag = "master"
+	}
+
+	if commitRegex.MatchString(tag) {
+		args["rev"] = tag
+	} else {
+		args["ref"] = tag
+	}
+
+	j, _ := json.Marshal(args)
+
+	return "git", string(j)
+}
+
+func (g *GitSource) CacheKey(pkgs []string, tag string) string {
+	// Only full commit hashes can be used for caching, as
+	// everything else is potentially a moving target.
+	if !commitRegex.MatchString(tag) {
+		return ""
+	}
+
+	unhashed := strings.Join(pkgs, "") + tag
+	hashed := fmt.Sprintf("%x", sha1.Sum([]byte(unhashed)))
+
+	return hashed
+}
+
+type NixChannel struct {
+	channel string
+}
+
+func (n *NixChannel) Render(tag string) (string, string) {
+	return "nixpkgs", n.channel
+}
+
+func (n *NixChannel) CacheKey(pkgs []string, tag string) string {
+	// Since Nix channels are downloaded from the nixpkgs-channels
+	// Github, users can specify full commit hashes as the
+	// "channel", in which case builds are cacheable.
+	if !commitRegex.MatchString(n.channel) {
+		return ""
+	}
+
+	unhashed := strings.Join(pkgs, "") + n.channel
+	hashed := fmt.Sprintf("%x", sha1.Sum([]byte(unhashed)))
+
+	return hashed
+}
+
+type PkgsPath struct {
+	path string
+}
+
+func (p *PkgsPath) Render(tag string) (string, string) {
+	return "path", p.path
+}
+
+func (p *PkgsPath) CacheKey(pkgs []string, tag string) string {
+	// Path-based builds are not currently cacheable because we
+	// have no local hash of the package folder's state easily
+	// available.
+	return ""
+}
+
+// Retrieve a package source from the environment. If no source is
+// specified, the Nix code will default to a recent NixOS channel.
+func pkgSourceFromEnv() (PkgSource, error) {
+	if channel := os.Getenv("NIXERY_CHANNEL"); channel != "" {
+		log.WithField("channel", channel).Info("using Nix package set from Nix channel or commit")
+
+		return &NixChannel{
+			channel: channel,
+		}, nil
+	}
+
+	if git := os.Getenv("NIXERY_PKGS_REPO"); git != "" {
+		log.WithField("repo", git).Info("using NIx package set from git repository")
+
+		return &GitSource{
+			repository: git,
+		}, nil
+	}
+
+	if path := os.Getenv("NIXERY_PKGS_PATH"); path != "" {
+		log.WithField("path", path).Info("using Nix package set at local path")
+
+		return &PkgsPath{
+			path: path,
+		}, nil
+	}
+
+	return nil, fmt.Errorf("no valid package source has been specified")
+}