Make 'sjar' and 'exclude_globally' real plugins

@jhump This is the change I was telling you about earlier today.

This code was migrated from 'generate_3rdparty.py' in our internal repo for the most part. I thought it might be more convenient for people to edit excludes in 'pants.yaml' at the root of the repo rather than what we had before, but I'm game for other ideas.