testonly artifacts from non-root modules no longer included

This appears to have been changed in #1511. This is a breaking change, as it means downstream root modules suddenly are missing targets they depend on.

The `testonly` attribute on maven.artifact dictates whether the generated Bazel target has the `testonly` attribute. Note that this is distinctly different than Maven's "test dependency" concept. It is expected that other tests and `testonly` BUILD targets in downstream modules can depend on ones generated from Maven artifacts defined in modules they depend on.

Even if this change were intentional and correct, it is a breaking change and should not have gone into a minor release.

Start by inspecting the change in #1511 and the handling of the maven.artifact testonly attribute for non-root modules. Reproduce a downstream root module depending on a generated testonly target; done when those targets remain available to downstream tests and BUILD targets without treating testonly as a Maven test dependency.

Written by the indexing model from the issue text.