Content root "runners/flink/1.5/src/main/java" is defined for modules "beam.runners.flink.1.5.main" and "beam.runners.flink.1.6.main".

When editing module settings in Intellij, I get the following error now:
```
Content root "~/git/beam/runners/flink/1.5/src/main/java" is defined for modules "beam.runners.flink.1.5.main" and "beam.runners.flink.1.6.main".
Two modules in a project cannot share the same content root.
```

I have hit this error before when getting "two" projects both point at the same source path. This has been worked around in the past by setting the source in compileJava/compileTestJava instead of updating the content roots to point to the same directory.

Both runners/google-cloud-dataflow-java/worker and runners/google-cloud-dataflow-java/worker/legacy-worker use the same source files. runners/google-cloud-dataflow-java/worker/legacy-worker uses a method that works around the Intellij limitation.