tpl/os: Revert readDir in theme behaviour

Fixes #9599

hugolib/filesystems/basefs.go

@@ -68,6 +68,9 @@ type BaseFs struct {
 	// This usually maps to /my-project/public.
 	PublishFs afero.Fs
 
+	// A read-only filesystem from the project workDir (no theme here).
+	WorkDir afero.Fs
+
 	theBigFs *filesystemsCollector
 
 	// Locks.
@@ -202,7 +205,7 @@ type SourceFilesystems struct {
 	// with any sub module's resource fs layered below.
 	ResourcesCache afero.Fs
 
-	// The project folder.
+	// The work folder (may be a composite of project and theme components).
 	Work afero.Fs
 
 	// When in multihost we have one static filesystem per language. The sync
@@ -435,9 +438,11 @@ func NewBase(p *paths.Paths, logger loggers.Logger, options ...func(*BaseFs) err
 
 	publishFs := hugofs.NewBaseFileDecorator(afero.NewBasePathFs(fs.Destination, p.AbsPublishDir))
 	sourceFs := hugofs.NewBaseFileDecorator(afero.NewBasePathFs(fs.Source, p.WorkingDir))
+	workDir := hugofs.NewBaseFileDecorator(afero.NewBasePathFs(afero.NewReadOnlyFs(fs.Source), p.WorkingDir))
 
 	b := &BaseFs{
 		SourceFs:  sourceFs,
+		WorkDir:   workDir,
 		PublishFs: publishFs,
 		buildMu:   lockedfile.MutexAt(filepath.Join(p.WorkingDir, lockFileBuild)),
 	}