private ZipEntry getEntry(JarFile jarFile, String dir) {
        if (!dir.endsWith("/")) {
            dir = dir + '/';
        }
        return jarFile.getEntry(dir + getConfigFilename(key));
    }
Returns the entry corresponding to the ConfigInstance's defName in the given directory in
the given jar file.
The file's existence is checked elsewhere.
config/src/main/java/com/yahoo/config/subscription/impl/JarConfigSubscription.java