[scala3] Use standard project structure

[RustedBones]

Use a standard project structure. Would help with #499, to add a test/src/test/scalajvm test source folder for test3 project only

From

src/
  core/
  examples/
  test/
  test-enum/ <-- was no referenced by build

to

core/src/main/scala/magnolify1/
examples/src/main/scala/magnolify1/examples/
test/src/test/
  java/magnolify1/tests/
  scala/magnolify1/tests/

[adamw]

Thanks!