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/fakefs.go

@@ -724,10 +724,6 @@ func (*fakeFS) underlying() (Filesystem, bool) {
 	return nil, false
 }
 
-func (*fakeFS) wrapperType() filesystemWrapperType {
-	return filesystemWrapperTypeNone
-}
-
 func (fs *fakeFS) resetCounters() {
 	fs.mut.Lock()
 	fs.counters = fakeFSCounters{}