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/walkfs.go

@@ -153,7 +153,3 @@ func (f *walkFilesystem) Walk(root string, walkFn WalkFunc) error {
 func (f *walkFilesystem) underlying() (Filesystem, bool) {
 	return f.Filesystem, true
 }
-
-func (*walkFilesystem) wrapperType() filesystemWrapperType {
-	return filesystemWrapperTypeWalk
-}