WIP: use shared symlink pkg cache on Windows newer than XP

This is following up on something @kmsquire asked me to do in #7274, now that symlinks sort of work on Windows newer than XP. Symlinks to files require admin rights in Vista/7/8, so for symlink to a directory we went with NTFS junctions instead.

Putting this up for review first, I'd like to do a bit more testing and verify that there are no bad interactions with #7055 for NTFS junctions. Also see #4396 for earlier discussion.