rename package avian.avian_vm_resource to avian.avianvmresource

This package name must match the URL protocol we use for loading
embedded resources, but OpenJDK's URL class won't tolerate underscores
in a protocol name.  Also, I had not updated the names of the native
methods in avian.avianvmresource.Handler, leading to
UnsatisfiedLinkErrors when they were called.

src/finder.cpp

@@ -602,7 +602,7 @@ class BuiltinElement: public JarElement {
   }
 
   virtual const char* urlPrefix() {
-    return "avian_vm_resource:";
+    return "avianvmresource:";
   }
 
   virtual const char* sourceUrl() {