[1.21.3] Add method to allow building custom DataGenerator.PackGenerator instances

[ApexModder]

This pr adds a simple utility method to DataGenerator getPackGenerator which allows constructing custom PackGenerator instances.

Currently the only way to generate a builtin pack is by using getBuiltinDatapack which generates to ./data/<namespace>/datapacks/<path>.
This change allows modders to override this and output their builtin packs to any location they choose, relative to the root pack directory.

Example use case would be generating a builtin resource pack or feature pack (which contains both client and server resources).

[neoforged-pr-publishing]

• Publish PR to GitHub Packages

Last commit published: b94e484d87a8f6bf76a79ec33a7678bf3bac0e08.

PR Publishing

The artifacts published by this PR:

• 📦 net.neoforged:neoforge:21.3.3-beta-pr-1624-pr-pack-generator-ctor
• 📦 net.neoforged:testframework:21.3.3-beta-pr-1624-pr-pack-generator-ctor

Repository Declaration

In order to use the artifacts published by the PR, add the following repository to your buildscript:

repositories {
    maven {
        name 'Maven for PR #1624' // https://github.com/neoforged/NeoForge/pull/1624
        url 'https://prmaven.neoforged.net/NeoForge/pr1624'
        content {
            includeModule('net.neoforged', 'neoforge')
            includeModule('net.neoforged', 'testframework')
        }
    }
}

MDK installation

In order to setup a MDK using the latest PR version, run the following commands in a terminal.
The script works on both *nix and Windows as long as you have the JDK bin folder on the path.
The script will clone the MDK in a folder named NeoForge-pr1624.
On Powershell you will need to remove the -L flag from the curl invocation.

mkdir NeoForge-pr1624
cd NeoForge-pr1624
curl -L https://prmaven.neoforged.net/NeoForge/pr1624/net/neoforged/neoforge/21.3.3-beta-pr-1624-pr-pack-generator-ctor/mdk-pr1624.zip -o mdk.zip
jar xf mdk.zip
rm mdk.zip || del mdk.zip

To test a production environment, you can download the installer from here.