fs: remove os.FileInfo from fs.ExtendedFileInfo

Only the `Sys()` value from os.FileInfo is kept as field `sys` to
support Windows. The os.FileInfo removal ensures that for values like
`ModTime` that existed in both data structures there's no more confusion
which value is actually used.

internal/archiver/scanner_test.go

@@ -57,7 +57,7 @@ func TestScanner(t *testing.T) {
 				},
 			},
 			selFn: func(item string, fi *fs.ExtendedFileInfo, fs fs.FS) bool {
-				if fi.IsDir() {
+				if fi.Mode.IsDir() {
 					return true
 				}