Use exclusion instead of inclusion in shadowJar

[vorburger]

It would probably be preferable to use exclusion instead of inclusion in shadowJar.

Instead of exclude, dependencies exclude is probably what we would want.

A first attempt I did that caused:

java.lang.IllegalArgumentException: null
        at org.objectweb.asm.ClassReader.<init>(Unknown Source) ~[spongevanilla-1.12.2-7.3.1-RC391.jar:1.12.2-7.3.1-RC391]
        at org.objectweb.asm.ClassReader.<init>(Unknown Source) ~[spongevanilla-1.12.2-7.3.1-RC391.jar:1.12.2-7.3.1-RC391]
        at org.objectweb.asm.ClassReader.<init>(Unknown Source) ~[spongevanilla-1.12.2-7.3.1-RC391.jar:1.12.2-7.3.1-RC391]
        at org.spongepowered.server.launch.plugin.PluginScanner.scanClassFile(PluginScanner.java:341) ~[spongevanilla-1.12.2-7.3.1-RC391.jar:1.12.2-7.3.1-RC391]

which seems to mean a Java >1.8 JAR snuck in.

The best way to do is probably to compare the Web fat JAR currently built with the one built with a change for this.