Add Automatic-Module-Name for XNIO Modules

If you're creating a Java module and using Gradle as your build system, you will get a "module not found" error for this line in module-info.java:

requires xnio.api;

The root cause is that implied automatic modules are not included on the module path in Gradle. The easiest fix is just to add "Automatic-Module-Name" to MANIFEST.MF. (You could also add a module-info.java file, but that's a more involved fix.)

(This issue does not affect Maven builds, though; Maven includes implied automatic modules on the module path.)

AFAIK, this change should be applied to the xnio-api and xnio-nio modules. (Are there other modules that this should be applied to?) We'll just use the current module name as the Automatic-Module-Name.

For more context, see https://github.com/gradle/gradle/issues/26726