[1.21] Fix data generator merge issue

[ApexModder]

When using the current DataGenerator system to generate built-in packs, you can easily run into Duplicate Provider errors.

This is due to how the system merges all providers down to a single larger provider in GatherDataEvent.
The current implementation ignores the id providers are registered with, which for most providers is no issue, but providers registered for built-in packs have the id prefixed with the packs id.

this PR aims to fix that issue by introducing a simple DataGenerator.merge method, which simply pulls the providers and their registered ids directly from the sub generators backing maps and moving them into the parents.

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This pr also introduces a simple overload helper DataGenerator.getBuiltinDatapack, this is to allow generating modded builtin packs.

Original Discord message

[neoforged-pr-publishing]

• Publish PR to GitHub Packages

Last commit published: 04fba14480e149f372568ebbaf08e3ed1405b3e6.

PR Publishing

The artifacts published by this PR:

• 📦 net.neoforged:neoforge:21.0.40-beta-pr-1187-pr-data-gen-issue
• 📦 net.neoforged:testframework:21.0.40-beta-pr-1187-pr-data-gen-issue

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 #1187' // https://github.com/neoforged/NeoForge/pull/1187
        url 'https://prmaven.neoforged.net/NeoForge/pr1187'
        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-pr1187.
On Powershell you will need to remove the -L flag from the curl invocation.

mkdir NeoForge-pr1187
cd NeoForge-pr1187
curl -L https://prmaven.neoforged.net/NeoForge/pr1187/net/neoforged/neoforge/21.0.40-beta-pr-1187-pr-data-gen-issue/mdk-pr1187.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.