Only include **/*.java in Java source directories

Since Maven only recognizes *.java files in Java source directories (src/main/java, or src/test/java, for example), it would be nice if the plugin produced .classpath files configured the same way. Right now, a developer can inadvertently leave a non-Java resource in one of those directories, and everything will work fine in Eclipse. But when the time comes to produce a package, that artifact will be missing since Maven won't pick it up.