Fix only rule config issues

[mildm8nnered]

WIP

Partly addresses #5711

func merged(with child: RulesWrapper) -> RulesWrapper is a problem, as this will not respect --only-rules - we may need a dedicated onlyRule RulesMode to get this to work in all cases.

[SwiftLintBot]

	1 Warning
⚠️	If this is a user-facing change, please include a CHANGELOG entry to credit yourself! You can find it at CHANGELOG.md.

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Here's an example of your CHANGELOG entry:

* Fix only rule config issues.  
  [mildm8nnered](https://github.com/mildm8nnered)
  [#issue_number](https://github.com/realm/SwiftLint/issues/issue_number)

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[mildm8nnered]

Replaced by #5773