Fix packages links after web_ui compile

I've further extended the buildpack to include fixing all `packages` symlinks after build.dart is executed.

The `web/out` folder (at least) contains several links to `packages` after the web_ui compiler runs, and these need fixing up before the slug will work on Heroku.

I also have set the path to include the dart bin directory before `build.dart` is executed, since I run `dart2js` from my build.dart.

In the case where no `build.dart` is present, these changes should not affect the buildpack's behaviour, except to fix any extra links to `packages` if they are not at `web/packages`.