chore(internal/provider): refactor cached_image_resource (#46)

Addresses some non-blocking comments from #44:

- Extracts some of the functions in cached_image_resource.go to separate internal packages tfutil and imgutil.
- Some other functions are extracted to helpers.go.
- Extracts non-overridable flags to a package-level variable.
- Pre-allocates some slices where possible.
- Removes some unused code and renames some existing code for readability

internal/imgutil/imgutil.go

@@ -0,0 +1,103 @@
+package imgutil
+
+import (
+	"archive/tar"
+	"context"
+	"fmt"
+	"io"
+	"os"
+	"path/filepath"
+
+	"github.com/coder/envbuilder/constants"
+	"github.com/google/go-containerregistry/pkg/authn"
+	"github.com/google/go-containerregistry/pkg/name"
+	v1 "github.com/google/go-containerregistry/pkg/v1"
+	"github.com/google/go-containerregistry/pkg/v1/remote"
+	"github.com/hashicorp/terraform-plugin-log/tflog"
+)
+
+// GetRemoteImage fetches the image manifest of the image.
+func GetRemoteImage(imgRef string) (v1.Image, error) {
+	ref, err := name.ParseReference(imgRef)
+	if err != nil {
+		return nil, fmt.Errorf("parse reference: %w", err)
+	}
+
+	img, err := remote.Image(ref, remote.WithAuthFromKeychain(authn.DefaultKeychain))
+	if err != nil {
+		return nil, fmt.Errorf("check remote image: %w", err)
+	}
+
+	return img, nil
+}
+
+// ExtractEnvbuilderFromImage reads the image located at imgRef and extracts
+// MagicBinaryLocation to destPath.
+func ExtractEnvbuilderFromImage(ctx context.Context, imgRef, destPath string) error {
+	needle := filepath.Clean(constants.MagicBinaryLocation)[1:] // skip leading '/'
+	img, err := GetRemoteImage(imgRef)
+	if err != nil {
+		return fmt.Errorf("check remote image: %w", err)
+	}
+
+	layers, err := img.Layers()
+	if err != nil {
+		return fmt.Errorf("get image layers: %w", err)
+	}
+
+	// Check the layers in reverse order. The last layers are more likely to
+	// include the binary.
+	for i := len(layers) - 1; i >= 0; i-- {
+		ul, err := layers[i].Uncompressed()
+		if err != nil {
+			return fmt.Errorf("get uncompressed layer: %w", err)
+		}
+
+		tr := tar.NewReader(ul)
+		for {
+			th, err := tr.Next()
+			if err == io.EOF {
+				break
+			}
+
+			if err != nil {
+				return fmt.Errorf("read tar header: %w", err)
+			}
+
+			name := filepath.Clean(th.Name)
+			if th.Typeflag != tar.TypeReg {
+				tflog.Debug(ctx, "skip non-regular file", map[string]any{"name": name, "layer_idx": i + 1})
+				continue
+			}
+
+			if name != needle {
+				tflog.Debug(ctx, "skip file", map[string]any{"name": name, "layer_idx": i + 1})
+				continue
+			}
+
+			tflog.Debug(ctx, "found file", map[string]any{"name": name, "layer_idx": i + 1})
+			if err := os.MkdirAll(filepath.Dir(destPath), 0o755); err != nil {
+				return fmt.Errorf("create parent directories: %w", err)
+			}
+			destF, err := os.Create(destPath)
+			if err != nil {
+				return fmt.Errorf("create dest file for writing: %w", err)
+			}
+			defer destF.Close()
+			_, err = io.Copy(destF, tr)
+			if err != nil {
+				return fmt.Errorf("copy dest file from image: %w", err)
+			}
+			if err := destF.Close(); err != nil {
+				return fmt.Errorf("close dest file: %w", err)
+			}
+
+			if err := os.Chmod(destPath, 0o755); err != nil {
+				return fmt.Errorf("chmod file: %w", err)
+			}
+			return nil
+		}
+	}
+
+	return fmt.Errorf("extract envbuilder binary from image %q: %w", imgRef, os.ErrNotExist)
+}