make clear that scope is ignored purposely in FileTask.scope_name

[rickhull]

I'm not 100% certain on whether this change is desirable or self-evidently so. But I definitely had to do a double-take when I looked at the method definition and saw that only the last argument was even considered. The existing comments somewhat explain it. I think the leading underscore on _scope makes the code more self-evident.

Note, this PR was resubmitted from a non-master branch of my fork.