Idiomatize(?) ruleset package and run lint

[dxbednarczyk]

Fixes a usability issue in the ruleset package where the original CLI flags are used in every function instead of creating a file to write to and passing around its io.Writer.

Also runs make lint and fixes most things that golangci-lint complains about.

[mms-gianni]

I'll need a moment to check this.

The rulesets are on the way out to a separate repository. but I'll merge it here and then migrate it to the new repository.

[ladddder]

This breaks the functionality of RULESET=rulesets go run cmd/main.go --merge-rulesets which was printing to stdout

But this required in future build jobs in the ladder-rules repository.

[dxbednarczyk]

This breaks the functionality of RULESET=rulesets go run cmd/main.go --merge-rulesets which was printing to stdout

The latest commit defaults to os.Stdout if mergeRulesetsOutput is empty. Can you confirm if it fixes the issue?

[ladddder]

@dxbednarczyk Works perfectly now.

Tested the other 2 parameters too.