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/builder/archive.go

@@ -1,116 +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 builder
-
-// This file implements logic for walking through a directory and creating a
-// tarball of it.
-//
-// The tarball is written straight to the supplied reader, which makes it
-// possible to create an image layer from the specified store paths, hash it and
-// upload it in one reading pass.
-
-import (
-	"archive/tar"
-	"compress/gzip"
-	"crypto/sha256"
-	"fmt"
-	"io"
-	"os"
-	"path/filepath"
-
-	"github.com/google/nixery/server/layers"
-)
-
-// Create a new compressed tarball from each of the paths in the list
-// and write it to the supplied writer.
-//
-// The uncompressed tarball is hashed because image manifests must
-// contain both the hashes of compressed and uncompressed layers.
-func packStorePaths(l *layers.Layer, w io.Writer) (string, error) {
-	shasum := sha256.New()
-	gz := gzip.NewWriter(w)
-	multi := io.MultiWriter(shasum, gz)
-	t := tar.NewWriter(multi)
-
-	for _, path := range l.Contents {
-		err := filepath.Walk(path, tarStorePath(t))
-		if err != nil {
-			return "", err
-		}
-	}
-
-	if err := t.Close(); err != nil {
-		return "", err
-	}
-
-	if err := gz.Close(); err != nil {
-		return "", err
-	}
-
-	return fmt.Sprintf("sha256:%x", shasum.Sum([]byte{})), nil
-}
-
-func tarStorePath(w *tar.Writer) filepath.WalkFunc {
-	return func(path string, info os.FileInfo, err error) error {
-		if err != nil {
-			return err
-		}
-
-		// If the entry is not a symlink or regular file, skip it.
-		if info.Mode()&os.ModeSymlink == 0 && !info.Mode().IsRegular() {
-			return nil
-		}
-
-		// the symlink target is read if this entry is a symlink, as it
-		// is required when creating the file header
-		var link string
-		if info.Mode()&os.ModeSymlink != 0 {
-			link, err = os.Readlink(path)
-			if err != nil {
-				return err
-			}
-		}
-
-		header, err := tar.FileInfoHeader(info, link)
-		if err != nil {
-			return err
-		}
-
-		// The name retrieved from os.FileInfo only contains the file's
-		// basename, but the full path is required within the layer
-		// tarball.
-		header.Name = path
-		if err = w.WriteHeader(header); err != nil {
-			return err
-		}
-
-		// At this point, return if no file content needs to be written
-		if !info.Mode().IsRegular() {
-			return nil
-		}
-
-		f, err := os.Open(path)
-		if err != nil {
-			return err
-		}
-
-		if _, err := io.Copy(w, f); err != nil {
-			return err
-		}
-
-		f.Close()
-
-		return nil
-	}
-}