JDK-8242596 improved multi-release JarFile lookup performance by reducing the number of versions searched from the full range [featureVersion - 9] to just the set of versions actually present in the JAR file.

However, the versions directory is generally very sparsly populated, meaning most JAR entries are not versioned. By tracking versions only for the few entries which are actually versioned, we can skip version-based lookup for non-versioned entries.