The extension build system only copies class files in the same package (plus any subpackages) of the extension. Because you're using Java files in a separate package, they won't be picked up by the extension system. The easiest thing to do would be to compile the dependency Java files to get the class files and put them into a JAR file (using the zip or jar utilities) and then place that in lib and make use of the library functionality built into the extension system since it knows how to pick up JAR libraries via the @UsesLibraries annotation.
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