Update and correct macro cross tutorial

[som-snytt]

Fixes https://github.com/scala/scala3/issues/20591

Besides the use of jpath, the scala 2 macro impl must be public. I tried to verify the steps as presented, but did not quite have the patience; probably the sophisticated user will fill in any gaps.

It's hard not to turn it into an sbt tutorial.

It's nicer just to say + test.