rule's "scalacopts" should get higher priority than toolchain "scalacopts"

In `scalacopts` there are some options that can be overridden by their last occurrance and some options that are aggregated

We need a test that makes sure that options that come from library rule `scalacopts` comes after any option that comes from toolchain's `scalacopts`

On #364 I did not add such test and I think it should be added.

Locate the scalacopts handling and review the test changes from #364, then trace how library-rule and toolchain options are ordered. Add a test that verifies library rule scalacopts occur after toolchain scalacopts, including options whose last occurrence wins and options that aggregate.

Written by the indexing model from the issue text.