Add Skylint checks to CI

It would be helpful to avoid deprecation surprises and maintain code quality to run [Skylint](https://github.com/bazelbuild/bazel/blob/master/site/docs/skylark/skylint.md) on `rules_scala` in CI.

Right now we have [39 warnings](https://github.com/bazelbuild/rules_scala/files/1465260/skylint.txt) that we would need to address before doing so. Should we make tickets for each group of warnings? Or work on them all in this issue?

Begin with the attached skylint.txt warning list and the linked Skylint documentation. Group the 39 warnings, then inspect the repository's CI configuration to determine where the check belongs. Done means the warnings are addressed and Skylint runs successfully in CI.

Written by the indexing model from the issue text.