Apparently, you need to use a directory junction. This feature is scarcely documented and has important drawbacks you need to know (check Wikipedia).
- Start cmd session as admin
- Navigate to Packages
- Move Packages to desired location (same volume, see Wikipedia)
- mklink /J User x:\path\to\User
This will kind of work, but in my brief testing it trips up Sublime Text file system watcher. (Or maybe it isn't posible to monitor changes for reparse points, I don't know.)
Be sure to remove it carefully if you need to, or you may end up deleting important files. Safest way seems to be like this:
fsutil.exe reparsepoint delete User
... which funnily enough, will leave an empty User directory behind.
Judging by the obscure documentation on this feature, it looks like somebody doesn't want you to use it.
Tested on Windows 7 Ultimate