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
I think we can PROBABLY skip the readlinking here, and instead just do the logic of "if packagePath exists (files.exists), then add packagePath to buildTempDirs".  I don't think we necessarily have to read the isopack not via the symlink.