Refactor symlinks for tropohouse and tool

In order to sanely replace the symlink code on Windows, first we need to factor it out into as few places as possible.

1. Now we use the isopack.json file (read with the logic from isopack.js) to find out which arches are downloaded
2. There is a function specifically for making the latest tool symlink, which will be replaced with a non-symlink solution on Windows
The second half of this comment is no longer true, right?

But instead, there should probably be a new comment explaining that previous versions of meteor used the link structure to decide what to download, and unfortunately unless we want to split off a new packages (eg .meteor/packages/v2) or break the ability to use older versions simultaneously, we're sort of stuck with it.  Right?