CSS Checker

We need a Java program that plugs into the CSS compiler (just like [JsChecker](https://github.com/bazelbuild/rules_closure/blob/master/java/com/google/javascript/jscomp/JsChecker.java)) which will do two things:
1. Strict dependency checking: Validate dependent rules form a transitive closure.
2. Output a `rule-provided.txt` file containing a `css:foo` for each `.foo { ... }` which can be used by JsChecker to validate `goog.getCssName('foo')` statements.

CC: @iflan