internal: Fix a severe bug with the file-system wrappers on OSX. `os.walk` is expected to support modifying its return values `dirnames` and `filenames` so that you can filter out items that need to be iterated upon. I was previously returning a different list in the wrappers, which means filtering was completely broken.

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title: The Whole Kitchen Burned Down!
layout: error
---
Something critically bad happened, and **PieCrust** needs to shut down. It's probably our fault.