chore(fs): changes to allow Filesystem to be implemented externally (#10040)

### Purpose

The `fs.Filesystem` interface contains two parts that cannot be
implemented externally because they are private:

* `filesystemWrapperType`: this PR changes `unwrapFilesystem` to
downcast to a specific concrete type
* `underlying`: this PR simply moves it to an unexported interface

### Testing

Regular tests pass.

lib/fs/mtimefs_test.go

@@ -261,7 +261,7 @@ func newMtimeFS(path string, db database, options ...MtimeFSOption) *mtimeFS {
 
 func newMtimeFSWithWalk(path string, db database, options ...MtimeFSOption) (*mtimeFS, *walkFilesystem) {
 	fs := NewFilesystem(FilesystemTypeBasic, path, NewMtimeOption(db, options...))
-	wfs, _ := unwrapFilesystem(fs, filesystemWrapperTypeWalk)
-	mfs, _ := unwrapFilesystem(fs, filesystemWrapperTypeMtime)
-	return mfs.(*mtimeFS), wfs.(*walkFilesystem)
+	wfs, _ := unwrapFilesystem[*walkFilesystem](fs)
+	mfs, _ := unwrapFilesystem[*mtimeFS](fs)
+	return mfs, wfs
 }