Refactor Package structure

This commit cleans up the package structure, to make two new top-level
packages `internal` (for internal-use packages) and `pkg` (for general
shared utility code).

pkg/targz/targz.go

@@ -0,0 +1,80 @@
+// Package targz provides facilities for packaging tar.gz files
+package targz
+
+import (
+	"archive/tar"
+	"compress/gzip"
+	"io"
+	"io/fs"
+	"os"
+	"path/filepath"
+)
+
+// Package packages the source directory into a 'tar.gz' file into destination.
+// If the destination already exists, it is truncated.
+//
+// onCopy, when not nil, is called for each file being copied into the archive.
+func Package(dst, src string, onCopy func(rel string, src string)) (count int64, err error) {
+	// create the target archive
+	archive, err := os.Create(dst)
+	if err != nil {
+		return 0, err
+	}
+	defer archive.Close()
+
+	// create a gzip writer
+	zipHandle := gzip.NewWriter(archive)
+	defer zipHandle.Close()
+
+	// create a tar writer
+	tarHandle := tar.NewWriter(zipHandle)
+	defer tarHandle.Close()
+
+	// and walk through it!
+	err = filepath.Walk(src, func(path string, info fs.FileInfo, err error) error {
+		if err != nil {
+			return err
+		}
+
+		// determine the relative path
+		var relpath string
+		relpath, err = filepath.Rel(src, path)
+		if err != nil {
+			return err
+		}
+
+		if onCopy != nil {
+			onCopy(relpath, path)
+		}
+
+		// create a file info header!
+		tInfo, err := tar.FileInfoHeader(info, relpath)
+		if err != nil {
+			return err
+		}
+		tInfo.Name = filepath.ToSlash(relpath)
+
+		// write it!
+		if err := tarHandle.WriteHeader(tInfo); err != nil {
+			return err
+		}
+
+		// a directory => no more writing required
+		if info.IsDir() {
+			return nil
+		}
+
+		// open the file
+		handle, err := os.Open(path)
+		if err != nil {
+			return err
+		}
+		defer handle.Close()
+
+		// and copy it into the archive
+		ccount, err := io.Copy(tarHandle, handle)
+		count += ccount
+		return err
+	})
+	return
+}