Make rules_closure conform to Style Guide for Rules

[Yannic]

Tracking bug for making rules_closure conform to Bazel's Style Guide for Rules.

Known issues:

• [x] Dependencies / Toolchains should be declared by rules_closure_{dependencies,toolchains} in closure/repositories.bzl: https://docs.bazel.build/versions/master/skylark/deploying.html#dependencies
• [ ] Tests should live in //tests
• [x] Repo description should be Closure rules for Bazel
• [ ] Workspace should be named rules_closure (Note: There are probably too many existing users now, so this is going to be a wontFix issue).

[Yannic]

@alexeagle Can you handle the repo description issue?

[alexeagle]

Sure, done, thanks for picking this up.

[Yannic]

Thank you!