The system and extension class loaders will require changes to work with a modular image.
As these class loaders are URLClassLoaders then a possible approach (that doesn't involve significant surgery) is to create a URL protocol handler that to support access for both class loading and resource files. This will likely be created to grant permissions too (see JDK-8047766).
A possible optimization is to seed these class loaders with the list of local packages so that they can avoid delegation. Minimally the extension class loader should not need to delegate for local packages unless -Xbootclasspath is set. Similarly the system class loader should not need to delegate unless there are 3rd party libraries in the ext directories (or -Xbootclasspath is set).