Update JarClassFileContainer to comply with new class URI structure

This is related to the issue javapathfinder#50 which is about JPF failing to load
model classes from jpf-classes.jar due to different URI structure of the
.class files being introuduced in Java Module System.

The previous workaround introudced by the PR javapathfinder#51 is specifically for the
jpf-classes.jar And JPF will still fail to locate classes from the other
jar files.

Thus this reverts it. And make a implementation changes to
JarClassFileContainer which is JPF's universal continer for loading
classes from any jar file.

src/main/gov/nasa/jpf/jvm/JarClassFileContainer.java

@@ -113,14 +113,15 @@ static String getPath(File file, String pathPrefix){
   }
 
   @Override
-  public ClassFileMatch getMatch(String clsName) throws ClassParseException {
-    String pn = clsName.replace('.', '/') + ".class";
+  public ClassFileMatch getMatch(String clsName) throws ClassParseException, ClassNotFoundException {
+    String moduleName = Class.forName(clsName).getModule().getName();
+    String entryName = moduleName + "/" + clsName.replace('.', '/') + ".class";
 
     if (pathPrefix != null){
-      pn = pathPrefix + pn;
+      entryName = pathPrefix + entryName;
     }
 
-    JarEntry e = jar.getJarEntry(pn);
+    JarEntry e = jar.getJarEntry(entryName);
 
     if (e != null) {
       InputStream is = null;