totally didn't realize `File.expand_path` would do this. still a bit new to ruby in general!
i implemented this, but then realized that it was almost exactly the same as the `else` condition, which reads:

    gemdir = File.expand_path(gemdir, caller_dir)

`caller_dir` for me is the absolute path to `./mruby/tasks`, which seems...wrong? there won't be any gems in there.