internal/pkg: reject entry types disallowed in the cache

These are not encoded in the format, they are rejected here to serve as a check for cache since checksum is computed for every directory.

Signed-off-by: Ophestra <cat@gensokyo.uk>

internal/pkg/dir.go

@@ -184,6 +184,8 @@ func Flatten(fsys fs.FS, root string, w io.Writer) (n int, err error) {
 				return err
 			}
 			ent.Data = []byte(newpath)
+		} else if !ent.Mode.IsDir() {
+			return InvalidFileModeError(ent.Mode)
 		}
 
 		nr, err = ent.Encode(w)