Pantsbuild: Introduce dependency rules (aka "visibility")

[cognifloyd]

Pants has a nifty feature that I've been looking forward to using in the st2 codebase called "visibility":

https://www.pantsbuild.org/2.19/docs/using-pants/validating-dependencies

Adding the dependency rules found an unexpected dependency from contrib/packs/pack.yaml to the test fixtures in contrib/packs, so there's one slight change to the pack_metadata pants plugin to fix that.