Add style guide rules about modules and requires

@spicyj @benkomalo @chrisklaiber 
unfortunately, our codebase is odd enough we should probably clarify what "third-party" means here.

strawman: I think anything under `third_party/` (and therefore has some node_modules/ entry) should be considered `third_party`. e.g. `react-components` is "third-party" under this definition, but `typeahead-vanilla-package/typeahead.js` is not.