feat: allow paths to be skipped by template evaluation

eg. The following file will be omitted if `.Include` is false.

```
-rw-r--r--  1 alec  staff   8 20 Nov 12:51 {{ if .Include }}include{{ end }}
```

README.md

@@ -7,5 +7,6 @@ ctx:
 
 - Both path names and file contents are evaluated.
 - If a file name ends with `.tmpl`, the `.tmpl` suffix is removed.
+- If a file or directory name evalutes to the empty string it will be excluded.
 
 [cookiecutter]: https://github.com/cookiecutter/cookiecutter

scaffolder.go

@@ -1,6 +1,7 @@
 package scaffolder
 
 import (
+	"errors"
 	"fmt"
 	"io/fs"
 	"os"
@@ -70,10 +71,17 @@ func Scaffold(source, destination string, ctx any, options ...Option) error {
 			return fmt.Errorf("failed to get file info: %w", err)
 		}
 
+		if lastComponent, err := evaluate(filepath.Base(path), ctx, opts.funcs); err != nil {
+			return fmt.Errorf("failed to evaluate path name: %w", err)
+		} else if lastComponent == "" {
+			return errSkip
+		}
+
 		dstPath, err := evaluate(path, ctx, opts.funcs)
 		if err != nil {
 			return fmt.Errorf("failed to evaluate path name: %w", err)
 		}
+
 		dstPath = filepath.Join(destination, dstPath)
 		dstPath = strings.TrimSuffix(dstPath, ".tmpl")
 
@@ -155,13 +163,19 @@ func applySymlinks(symlinks map[string]string, path string) error {
 	return os.Symlink(target, path)
 }
 
+// errSkip is returned by walkDir to skip a file or directory.
+var errSkip = errors.New("skip directory")
+
 // Depth-first walk of dir executing fn after each entry.
 func walkDir(dir string, fn func(path string, d fs.DirEntry) error) error {
 	dirInfo, err := os.Stat(dir)
 	if err != nil {
 		return err
 	}
 	if err = fn(dir, fs.FileInfoToDirEntry(dirInfo)); err != nil {
+		if errors.Is(err, errSkip) {
+			return nil
+		}
 		return err
 	}
 	entries, err := os.ReadDir(dir)
@@ -171,12 +185,12 @@ func walkDir(dir string, fn func(path string, d fs.DirEntry) error) error {
 	for _, entry := range entries {
 		if entry.IsDir() {
 			err = walkDir(filepath.Join(dir, entry.Name()), fn)
-			if err != nil {
+			if err != nil && !errors.Is(err, errSkip) {
 				return err
 			}
 		} else {
 			err = fn(filepath.Join(dir, entry.Name()), entry)
-			if err != nil {
+			if err != nil && !errors.Is(err, errSkip) {
 				return err
 			}
 		}

scaffolder_test.go

@@ -12,7 +12,8 @@ import (
 func TestScaffolder(t *testing.T) {
 	tmpDir := filepath.Join(t.TempDir(), "new")
 	err := Scaffold("testdata/template", tmpDir, map[string]any{
-		"Name": "test",
+		"Name":    "test",
+		"Include": true,
 	}, Exclude("excluded"))
 	assert.NoError(t, err)
 	type file struct {
@@ -21,6 +22,8 @@ func TestScaffolder(t *testing.T) {
 		content string
 	}
 	expect := []file{
+		{"include", 0o600, "included"},
+		{"included-dir/included", 0o600, "included"},
 		{"intermediate", 0o700 | os.ModeSymlink, "Hello, test!\n"},
 		{"regular-test", 0o600, "Hello, test!\n"},
 		{"symlink-test", 0o700 | os.ModeSymlink, "Hello, test!\n"},

testdata/template/{{ if .Include }}included-dir{{ end }}/included

@@ -0,0 +1 @@
+included

testdata/template/{{ if .Include }}include{{ end }}

@@ -0,0 +1 @@
+included