ddt clean always deletes the current libraries rlib

[MisterEggnog]

I was writing some basic tests for ddt clean. In doing so I also found a bug where it will always delete the current library's rlib, even in situations where the dependencies have not been changed.

I have included a test for this in #9.

[kdy1]

I'm sorry, but I'm closing this as I decided to drop ddt clean. Maybe I add similar feature in the future, but for now, I don't want to use very good name for such a narrow task.