tar.py: Don't import 'dev' nodes
Description
Dev nodes in tar files prevent extracting tar files, however, BuildStream's backend would not be able to do anything from it anyways. We should thus remove them anyways.
Discussion from the ML: https://lists.apache.org/thread.html/r92352e3e091d4e5806f0d09e6de86fd390053890cf82e2c7027495c7%40%3Cdev.buildstream.apache.org%3E