Add module-info.java file

[Sineaggi]

Add crac module for modular application support. Maybe use a multi-release jar so the module info file can be compiled on java 9+ and the base code can be compiled with 8.

[AntonKozlov]

Could elaborate on example where that would be helpful?

Indeed, we won't raise minimal requirements from Java 8, as for now, a Java 8 apps can run on JDK 17 CRaC and successfully coordinate with the checkpoint.

[Sineaggi]

So for example spring-framework uses org.crac.Resource, a class in this library to support stop-and-restart for spring. By adding a module-info.java, it would allow spring-context to be fully modularized.

As a stop-gap, it would be possible to add an Automatic-Module-Name to the jar's manifest to reserve a module name for the library, to avoid java from falling back to the jar name to derive an automatic module name.

FWIW, there are numerous libraries that use MR-jars to build a java-8 compatible core then place a module-info.java into the META-INF/versions/9 directory in the jar file. They would use java 8 (or higher) to compile 8-compatible code, then 9 (or higher) to compile the module-info.java file.