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/config.go

@@ -0,0 +1,84 @@
+// 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 implements structures to store Nixery's configuration at
+// runtime as well as the logic for instantiating this configuration from the
+// environment.
+package config
+
+import (
+	"os"
+
+	log "github.com/sirupsen/logrus"
+)
+
+func getConfig(key, desc, def string) string {
+	value := os.Getenv(key)
+	if value == "" && def == "" {
+		log.WithFields(log.Fields{
+			"option":      key,
+			"description": desc,
+		}).Fatal("missing required configuration envvar")
+	} else if value == "" {
+		return def
+	}
+
+	return value
+}
+
+// Backend represents the possible storage backend types
+type Backend int
+
+const (
+	GCS = iota
+	FileSystem
+)
+
+// Config holds the Nixery configuration options.
+type Config struct {
+	Port    string    // Port on which to launch HTTP server
+	Pkgs    PkgSource // Source for Nix package set
+	Timeout string    // Timeout for a single Nix builder (seconds)
+	WebDir  string    // Directory with static web assets
+	PopUrl  string    // URL to the Nix package popularity count
+	Backend Backend   // Storage backend to use for Nixery
+}
+
+func FromEnv() (Config, error) {
+	pkgs, err := pkgSourceFromEnv()
+	if err != nil {
+		return Config{}, err
+	}
+
+	var b Backend
+	switch os.Getenv("NIXERY_STORAGE_BACKEND") {
+	case "gcs":
+		b = GCS
+	case "filesystem":
+		b = FileSystem
+	default:
+		log.WithField("values", []string{
+			"gcs",
+		}).Fatal("NIXERY_STORAGE_BUCKET must be set to a supported value")
+	}
+
+	return Config{
+		Port:    getConfig("PORT", "HTTP port", ""),
+		Pkgs:    pkgs,
+		Timeout: getConfig("NIX_TIMEOUT", "Nix builder timeout", "60"),
+		WebDir:  getConfig("WEB_DIR", "Static web file dir", ""),
+		PopUrl:  os.Getenv("NIX_POPULARITY_URL"),
+		Backend: b,
+	}, nil
+}