Flag --incompatible_disable_target_provider_fields will break rules_scala in Bazel 1.2.1

Incompatible flag --incompatible_disable_target_provider_fields 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 with the linked Bazelisk incompatible-flags CI builds and reproduce the failures for --incompatible_disable_target_provider_fields across the listed Ubuntu, Darwin, and RBE environments. Trace the affected rules_scala behavior and confirm completion when rules_scala works with Bazel 1.2.1 and this flag enabled.

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