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/errorfs.go

@@ -69,7 +69,3 @@ func (fs *errorFilesystem) PlatformData(_ string, _, _ bool, _ XattrFilter) (pro
 func (*errorFilesystem) underlying() (Filesystem, bool) {
 	return nil, false
 }
-
-func (*errorFilesystem) wrapperType() filesystemWrapperType {
-	return filesystemWrapperTypeError
-}