Fix bug #7150 (Switching projects or quitting sometimes pauses for several seconds)

Fix bug #7150 -- Don't let Directory try to clear its immediate childrens' caches. If we've already visited the parent dir and then we visit one of its children, that child would have no _content anymore and would resort to a full index-walk to try to find its own immediate children. This is slow to do many times, and redundant since we're already doing a full walk anyway.