Flag --incompatible_disable_depset_items will break rules_scala in Bazel 1.2.1

Incompatible flag --incompatible_disable_depset_items will break rules_scala once Bazel 1.2.1 is released.

Please see the following CI builds for more information:

* [:ubuntu: 18.04 (OpenJDK 11)](:ubuntu: 18.04 (OpenJDK 11))
* [:ubuntu: 16.04 (OpenJDK 8)](:ubuntu: 16.04 (OpenJDK 8))
* [:darwin: (OpenJDK 8)](:darwin: (OpenJDK 8))
* [RBE (:ubuntu: 16.04, OpenJDK 8)](RBE (:ubuntu: 16.04, OpenJDK 8))

Questions? Please file an issue in https://github.com/bazelbuild/continuous-integration

**Important**: Please do NOT modify the issue title since that might break our tools.

Start by reviewing the linked Bazelisk incompatible-flags CI builds and the rules_scala compatibility entry points. Reproduce the failures with --incompatible_disable_depset_items against the Bazel 1.2.1 release target, then confirm that the rules_scala checks pass with the flag enabled.

Written by the indexing model from the issue text.