Allow extension system to load in custom OffensesFormatter

[alexevanczuk]

What are you trying to accomplish?

Today, packwerk supports a configurable OffensesFormatter. However, the only way to configure it is to load the CLI itself. This means that this feature is not usable if a user has run bundle binstubs packwerk. Instead, the user needs to copy exe/packwerk into their local bin/packwerk and pass in a different formatter to Packwerk::Cli.new.

This PR makes this more user-friendly by leveraging the extension system to load in a new formatter. This way, a user can use the vanilla packwerk binstub and load in their own offenses formatter more easily.

What approach did you choose and why?

I chose to use the require directive in packwerk.yml to load in a formatter via a setting on Packwerk::Configuration. The system first prioritizes the local input variable, then the configuration, then the default formatter.

What should reviewers focus on?

Is there a more ideal API to set the custom formatter? Is there any reason we wouldn't want to encourage this via the extension system?

Type of Change

• Bugfix
• New feature
• Non-breaking change (a change that doesn't alter functionality - i.e., code refactor, configs, etc.)

Checklist

• I have updated the documentation accordingly.
• I have added tests to cover my changes.
• It is safe to rollback this change.

[alexevanczuk]

I thought of an alternate implementation I think may be better. A --format option that matches on the name of a formatter. Then packwerk yml can specify the default. I'll put up a different PR later.