Work around --gc failure on Windows <= 10

This applies two related fixes/improvements:

* The --gc now keeps empty `_resources/_gen/images` etc folders, even if empty. This should have been the behaviour
from the start.
* Also, if removal of an empty dir on Windows fails with the "used by another process" error, just ignore it for now.

Fixes #10781

cache/filecache/filecache_pruner.go

@@ -17,6 +17,8 @@ import (
 	"fmt"
 	"io"
 	"os"
+	"runtime"
+	"strings"
 
 	"github.com/gohugoio/hugo/common/herrors"
 	"github.com/gohugoio/hugo/hugofs"
@@ -66,6 +68,7 @@ func (c *Cache) Prune(force bool) (int, error) {
 
 		if info.IsDir() {
 			f, err := c.Fs.Open(name)
+
 			if err != nil {
 				// This cache dir may not exist.
 				return nil
@@ -74,7 +77,24 @@ func (c *Cache) Prune(force bool) (int, error) {
 			_, err = f.Readdirnames(1)
 			if err == io.EOF {
 				// Empty dir.
-				err = c.Fs.Remove(name)
+				if name == "." {
+					// e.g. /_gen/images -- keep it even if empty.
+					err = nil
+				} else {
+					err = c.Fs.Remove(name)
+					if err != nil {
+						if runtime.GOOS == "windows" {
+							if strings.Contains(err.Error(), "used by another process") {
+								// See https://github.com/gohugoio/hugo/issues/10781
+								// This is a known issue on Windows with Go 1.20.
+								// There's not much we can do about it.
+								// So just return nil.
+								err = nil
+
+							}
+						}
+					}
+				}
 			}
 
 			if err != nil && !herrors.IsNotExist(err) {