Use app classloader instead of platform classloader as parent

When using the platform classloader as parent, service loaders fail to look up services that are defined in a jdk module.

As an example, consider the following onEnable of a plugin:

    @Override
    public void onEnable() {
        System.out.println(RandomGenerator.getDefault());
    }

this will result in an exception:

[12:41:12 ERROR]: Error occurred while enabling Paper-Test-Plugin v1.0.0-SNAPSHOT (Is it up to date?)
java.lang.IllegalArgumentException: No implementation of the random number generator algorithm "L32X64MixRandom" is available
        at java.util.random.RandomGeneratorFactory.findProvider(RandomGeneratorFactory.java:229) ~[?:?]
        at java.util.random.RandomGeneratorFactory.of(RandomGeneratorFactory.java:257) ~[?:?]
        at java.util.random.RandomGenerator.of(RandomGenerator.java:124) ~[?:?]
        at java.util.random.RandomGenerator.getDefault(RandomGenerator.java:139) ~[?:?]
        at io.papermc.testplugin.TestPlugin.onEnable(TestPlugin.java:12) ~[test-plugin-1.0.0-SNAPSHOT.jar:?]
        at org.bukkit.plugin.java.JavaPlugin.setEnabled(JavaPlugin.java:287) ~[paper-api-1.20.4-R0.1-SNAPSHOT.jar:?]
        at io.papermc.paper.plugin.manager.PaperPluginInstanceManager.enablePlugin(PaperPluginInstanceManager.java:188) ~[paper-1.20.4.jar:git-Paper-436]
        at io.papermc.paper.plugin.manager.PaperPluginManagerImpl.enablePlugin(PaperPluginManagerImpl.java:104) ~[paper-1.20.4.jar:git-Paper-436]
        at org.bukkit.plugin.SimplePluginManager.enablePlugin(SimplePluginManager.java:507) ~[paper-api-1.20.4-R0.1-SNAPSHOT.jar:?]
        at org.bukkit.craftbukkit.v1_20_R3.CraftServer.enablePlugin(CraftServer.java:639) ~[paper-1.20.4.jar:git-Paper-436]
        at org.bukkit.craftbukkit.v1_20_R3.CraftServer.enablePlugins(CraftServer.java:550) ~[paper-1.20.4.jar:git-Paper-436]
        at net.minecraft.server.dedicated.DedicatedServer.initServer(DedicatedServer.java:275) ~[paper-1.20.4.jar:git-Paper-436]
        at net.minecraft.server.MinecraftServer.runServer(MinecraftServer.java:1131) ~[paper-1.20.4.jar:git-Paper-436]
        at net.minecraft.server.MinecraftServer.lambda$spin$0(MinecraftServer.java:319) ~[paper-1.20.4.jar:git-Paper-436]
        at java.lang.Thread.run(Thread.java:1583) ~[?:?]

This is because the service loader doesn't find the proper implementations with the platform classloader being the direct parent of the context classloader. With this change, the service loader can properly look up the providers in the jdk.random module.

The behavior prior and after the change is the same on Java 17 and Java 21 respectively.

Some alternatives:

Plugins set the context classloader to the app classloader before accessing code that loads services from a jdk module
Plugins use reflections to load such services
Instead of using a URLClassLoader, use a custom ModuleLayer and load the jars as modules
Preload known services before calling the main method of the server

PaperMC / Paperclip

Use app classloader instead of platform classloader as parent #66