Revert r78903, r78904, r78905, r78906 (changing opendir() to use sfFinder class)...

Revert r78903, r78904, r78905, r78906 (changing opendir() to use sfFinder class) per CR and wikitech-l objections.
It's arguable whether the code is cleaner either way, and adding all this code for the (relatively few) places we do file listings just isn't worth it.