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.

---
in:
    pages/foo.md: |
        Foo!
    pages/bar.md: |
        Bar!
    pages/allpages.md: |
        {% for p in site.pages -%}
        {{p.url}}
        {% endfor %}
outfiles:
    allpages.html: |
        /
        /allpages.html
        /bar.html
        /foo.html