Make rules_closure conform to Style Guide for Rules

Tracking bug for making rules_closure conform to Bazel's [Style Guide for Rules](https://docs.bazel.build/versions/master/skylark/deploying.html).

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).

Read the Bazel Style Guide for Rules linked in the issue and inspect the repository layout, including closure/repositories.bzl and the current test locations. Determine what remains for tests to live in //tests, while noting that the workspace rename is marked likely wontFix. Done means the applicable style-guide items are addressed or explicitly excluded.

Written by the indexing model from the issue text.