This is investigation I've done in support of https://github.com/jruby/jruby/issues/6608 where I noticed some funky issues with JRuby and module loading. Looks like that JRuby expects JAVA_HOME to have a directory called `jmod`, which is consistent with the Java Module system. Unfortunately, the top level directory for the `jre` or `jdk` /nix/store entry is not a valid JAVA_HOME since it is missing that directory. Instead it's set within `lib/openjdk`, and there is a passthru variable set accordingly. This fixes JRuby and follows many other derivations. A simple search in the code-base shows that there are many other packages that suffer this same bug.