SemanticDB requires private fields of case class to use "val"

[jackkoenig]

This does not seem to be required by Scala, but it is required by newer versions of SemanticDB (and thus to use Scala Metals).

Workaround for scalameta/scalameta#3122.

Contributor Checklist

• Did you add Scaladoc to every public function/method?
• Did you add at least one test demonstrating the PR?
• Did you delete any extraneous printlns/debugging code?
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Type of Improvement

• Internal or build-related (includes code refactoring/cleanup)

Desired Merge Strategy

• Squash

Release Notes

Reviewer Checklist (only modified by reviewer)

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• Did you do one of the following when ready to merge:
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