zulint will be added as a "third-party" dependency in zulip from now on. See the new project at https://github.com/zulip/zulint for more details.
This makes linting rules in zulint more general. Make necessary changes in tools/lint and tools/custom_check.py to run with the new RuleList class. Modify tests for `RuleList` class. Tests only include minor changes to test with the new class.