Fig does not resolve @ in paths when using an unpublished package.fig

```
$ cat package.fig 
resource "dist/foo.jar"
config default 
  append CLASSPATH=@/dist/foo.jar
end

$ fig -- echo \$CLASSPATH
@/dist/foo.jar
```

Expected result:
    dist/foo.jar

Having this feature would make it really convenient to use fig for dependency management, without having to publish to a repository.
