"kpm pack" nupkg the project dependencies

parent #80 

@loudej mentioned in #80 that "source packages should be built innermost-first from a dependency standpoint". However, I find we can successfully build all project dependencies without considering the order. So in my current implementation, I just build project dependencies in an arbitrary order and copy the output nupkgs to final destination. @loudej , please let me know if you have any concern regarding this implementation.