Reveal internal but public identifiers and make them `private[cats]`

[satorg]

There are several classes or other identifiers across Cats that were made public initially even though they are not supposed to be accessible outside of the library:

• cats.kernel.instances.SeqMonoid - should only be exposed as Monoid[Seq] (see initial discussion)
• TBD

In order to simplify the migration and alleviate toil for users, it makes sense to consider a corresponding scalafix rule for that.

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_{The issue is created just to don't forget about it.}