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/casefs.go

@@ -357,10 +357,6 @@ func (f *caseFilesystem) underlying() (Filesystem, bool) {
 	return f.Filesystem, true
 }
 
-func (*caseFilesystem) wrapperType() filesystemWrapperType {
-	return filesystemWrapperTypeCase
-}
-
 func (f *caseFilesystem) checkCase(name string) error {
 	var err error
 	if name, err = Canonicalize(name); err != nil {