Flag --incompatible_disable_depset_items will break Bazel skylib in a future Bazel release

Incompatible flag --incompatible_disable_depset_items will be enabled by default in a future Bazel release [1], thus breaking Bazel skylib.

The flag is documented here: https://github.com/bazelbuild/bazel/issues/9017

Please check the following CI builds for build and test results:

* Ubuntu 18.04, OpenJDK 11
* Ubuntu 16.04, OpenJDK 8
* Windows, OpenJDK 8
* macOS, OpenJDK 8

Never heard of incompatible flags before? We have [documentation](https://docs.bazel.build/versions/master/backward-compatibility.html) that explains everything.

If you don't want to receive any future issues for Bazel skylib or if you have any 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.

[1] The target release hasn't been determined yet. Our tool will update the issue title once the flag flip has been scheduled.

Start by reading the incompatible flag documentation and reviewing the four linked CI builds for the build and test failures described. Inspect the Bazel skylib failures caused by --incompatible_disable_depset_items; done means the listed Ubuntu, Windows, and macOS builds pass with the flag enabled.

Written by the indexing model from the issue text.