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/basicfs.go

@@ -336,10 +336,6 @@ func (*BasicFilesystem) underlying() (Filesystem, bool) {
 	return nil, false
 }
 
-func (*BasicFilesystem) wrapperType() filesystemWrapperType {
-	return filesystemWrapperTypeNone
-}
-
 // basicFile implements the fs.File interface on top of an os.File
 type basicFile struct {
 	*os.File