Add ability for people to add custom pack finders (RepositorySources) (#8121)

Added AddPackFindersEvent which lets you add additional resource/data pack sources to the game.
Added PathResourcePack as a utility to allow easy providing of a subfolder in a mod as a resource pack.

src/main/java/net/minecraftforge/event/AddPackFindersEvent.java

@@ -0,0 +1,59 @@
+/*
+ * Minecraft Forge
+ * Copyright (c) 2016-2021.
+ *
+ * This library is free software; you can redistribute it and/or
+ * modify it under the terms of the GNU Lesser General Public
+ * License as published by the Free Software Foundation version 2.1
+ * of the License.
+ *
+ * This library is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
+ * Lesser General Public License for more details.
+ *
+ * You should have received a copy of the GNU Lesser General Public
+ * License along with this library; if not, write to the Free Software
+ * Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA  02110-1301  USA
+ */
+
+package net.minecraftforge.event;
+
+import net.minecraft.server.packs.PackType;
+import net.minecraft.server.packs.repository.RepositorySource;
+import net.minecraftforge.eventbus.api.Event;
+import net.minecraftforge.fml.event.IModBusEvent;
+
+import java.util.function.Consumer;
+
+/**
+ * Fired on PackRepository creation to allow mods to add new pack finders.
+ */
+public class AddPackFindersEvent extends Event implements IModBusEvent
+{
+    private final PackType packType;
+    private final Consumer<RepositorySource> sources;
+
+    public AddPackFindersEvent(PackType packType, Consumer<RepositorySource> sources)
+    {
+        this.packType = packType;
+        this.sources = sources;
+    }
+
+    /**
+     * Adds a new source to the list of pack finders.
+     * @param source the pack finder
+     */
+    public void addRepositorySource(RepositorySource source)
+    {
+        sources.accept(source);
+    }
+
+    /**
+     * @return the {@link PackType} of the resource manager being constructed.
+     */
+    public PackType getPackType()
+    {
+        return packType;
+    }
+}