fix(jandex): register the index as a sourceSet output directory

[vlsi]

Why

Under Gradle 9 the build fails whenever a task consumes a non-main sourceSet output without
declaring a dependency on processXxxJandexIndex:

Task ':useExtraOutput' uses this output of task ':processExtraJandexIndex' without declaring an
explicit or implicit dependency.

The plugin writes the index into sourceSet.output.resourcesDir, which makes
processXxxJandexIndex a second producer of that directory. It then relied on per-task dependsOn
wiring so consumers run after the copy. That wiring is broad for the main sourceSet but covers only
the jar and javadoc tasks for non-main sourceSets, so other consumers of the output — the JMH
bytecode generator, checkstyle, forbidden-apis — are left without the dependency. Gradle 9
turns that implicit dependency into a hard error. Reported downstream in pgjdbc/pgjdbc#4010.

What

• Build the index into a dedicated directory and register it as an extra output of the sourceSet:
  sourceSet.output.dir(mapOf("builtBy" to processJandexIndex), jandexResourcesDir). Gradle then
  wires every consumer of sourceSet.output to depend on processXxxJandexIndex automatically.
• Remove the manual dependsOn blocks for both the main and non-main sourceSets — they are now
  redundant, and they never covered consumers the plugin did not know about.
• The index still reaches the jar for BUILD_AND_INCLUDE, because the jar packages
  sourceSet.output, which now includes the registered directory.

How to verify

./gradlew :plugins:jandex-plugin:test

The new jandexDoesNotBreakNonMainSourceSetConsumers test pins Gradle 9, drives a non-main
sourceSet consumer, and runs across every JandexBuildAction mode (BUILD_AND_INCLUDE, BUILD,
VERIFY_ONLY, NONE), asserting the consumer builds and that the index reaches the jar only for
BUILD_AND_INCLUDE. Reverting the JandexPlugin.kt change makes that test fail with the
implicit_dependency validation error.