Add "fixJava8Classpath" option to support multirelease projects

[kohlschuetter]

In certain environments that build multirelease jars, the Maven "classes" directory contains class files with newer Java versions, whereas the Java 8 versions are stored elsewhere.

Add a boolean "fixJava8Classpath" option that, once enabled, automatically fixes these classpath entries. It checks for the presence of a sibling directory named "classes-java8". If that's available, then the original classpath entry is replaced.

[luontola]

Requires a test case to reproduce whatever was the issue. "classes-java8" sounds like something project specific.