fix: Add main sourceSet output to the inputs

[jmini]

Fixes #24

As discussed in the issue, the resolveSources() method is adding the main sourceSet output to the list of files sent to jandex.

jandex-gradle-plugin/src/main/groovy/org/kordamp/gradle/plugin/jandex/tasks/JandexTask.groovy

Lines 155 to 157 in 8b9f879

	if (resolvedProcessDefaultFileSet.get()) {
	files.addAll((Collection<String>) sourceSets.findByName('main').output.classesDirs*.absolutePath.flatten())
	}

So for me this belongs to the task input in order to avoid the task to stay UP-TO-DATE when there is a change.

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There was no guidance on functional tests in this repository.
So I used what is generated by gradle init as input but I did not used Spock.
If the setup is not the one that is expected, feel free to point me to other repositories where you have already functional tests.
Or if you prefer not having them, this can also be removed from this PR.

The test itself contains the reproducer provided by @jskillin-idt and makes sure that the outcome of the jandex task is correct (SUCCESS or UP_TO_DATE) depending if there was a source modification or not.

[Guliver]

I use the workaround as mentioned in #24 and I'm quite happy with it. It would be nice to have it as a default behaviour.

Is there any reason, why this PR don't get merged?