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

Incompatible flag --incompatible_no_implicit_file_export 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/10225

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 reviewing the linked Bazel incompatible-flag documentation and the four referenced CI builds for Ubuntu, Windows, and macOS. Done means Bazel skylib continues to build and pass its tests when --incompatible_no_implicit_file_export is enabled, but the issue does not identify the affected files or tests.

Written by the indexing model from the issue text.