IssueTracker

Users/third-party plugins may add sources to compile through the DSL, so sources that are collected during the configuration of the Java compile task may be incomplete.

So far we have worked around this issue by adding sources to the Java compile task also when the following DSL calls are made to register custom sources:
    - BaseVariantData.registerJavaGeneratingTask(...)
    - BaseVariantData.registerExternalAptJavaOutput(...)

The current approach has several issues:
    - It forces the Java compile task to be created and configured.
    - The custom sources are not added to the new ProcessAnnotationsTask. (Issue 115774858#comment6)
    - Sources may be added twice (which may slightly decrease performance).
   
To solve all of the issues above, we probably need to collect sources to compile at execution time instead.