Copy rules_directory to bazel-skylib.

[matts1]

Original implementation is at https://github.com/matts1/rules_directory

[matts1]

This is a rather large commit since it's just a copy-paste from rules_directory. I can split it up if that's required, but since that requires some effort on my part I'll just send it as one commit for now, and I can split it up if that's asked of me.