Allow to export a partially exported package globally when using preserveExisting

[jjohannes]

See: #178 (comment)

// original module-info contains: exports impl.org.controlsfx.skin to org.controlsfx.samples;
module("org.controlsfx:controlsfx", "controlsfx") {
    preserveExisting()
    exports("impl.org.controlsfx.skin")
}

Currently the above leads to an invalid module descriptor.

rror occurred during initialization of boot layer
java.lang.module.FindException: Error reading module: controlsfx-11.2.2-module.jar
Caused by: java.lang.module.InvalidModuleDescriptorException: Exported package impl.org.controlsfx.skin already declared

Instead the descriptor should only contain the added exports for impl.org.controlsfx.skin. The original one should be removed.

[koppor]

As discussed on slack, this might not be needed in the concrete case. The package names seem to have changed the last 5 years 😅