Tighten compatibility guarantees for semanticdb3

[xeno-by]

Because of the fact that ScalaPB generates case classes, semanticdb3 would violate our compatibility guarantees if it was made a public API. Problematic APIs include case class apply, unapply, copy and some other methods.

[olafurpg]

Looks like this is outside the scope of Scalameta since the fix involves contributing to ScalaPB.

[xeno-by]

Reopening this ticket to link it in the quickstart guide.