jrt-fs.jar is currently copied into the JDK image by the build. We need to re-examine this and consider whether it should be in java.base so that it is included in all runtime images (for cross compilation purposes with `javac -system`) or else moved to its own module (maybe `jdk.jrtfs`).

If jrt-fs.jar is included in all runtime images then it is likely that embedded users will want to exclude it when creating runtime images for small devices. This approach is also tied to the question on whether the JMOD format should have a "top-level" section.