Rewrite turtle upgrade modeller registration API

Originally we exposed a single registerTurtleUpgradeModellermethod which
could be called from both Fabric (during a mod's client init) and Forge
(during FMLClientSetupEvent).

This was fine until we allowed upgrades to specify model dependencies,
which would then automatically loaded, as this means model loading now
depends on upgrade modellers being loaded. Unknown to me, this is not
guaranteed to be the case on Forge - mod setup happens at the same time
as resource reloading!

Unfortunately there's not really a salvageable way of fixing this with
the current API. Forge now uses a registration event-based system,
meaning we can guarantee all modellers are loaded before models are
baked.

projects/common-api/src/client/java/dan200/computercraft/api/client/ComputerCraftAPIClient.java

@@ -27,8 +27,13 @@ public final class ComputerCraftAPIClient {
      * @param serialiser The turtle upgrade serialiser.
      * @param modeller   The upgrade modeller.
      * @param <T>        The type of the turtle upgrade.
+     * @deprecated This method can lead to confusing load behaviour on Forge. Use
+     * {@code dan200.computercraft.api.client.FabricComputerCraftAPIClient#registerTurtleUpgradeModeller} on Fabric, or
+     * {@code dan200.computercraft.api.client.turtle.RegisterTurtleModellersEvent} on Forge.
      */
+    @Deprecated(forRemoval = true)
     public static <T extends ITurtleUpgrade> void registerTurtleUpgradeModeller(TurtleUpgradeSerialiser<T> serialiser, TurtleUpgradeModeller<T> modeller) {
+        // TODO(1.20.4): Remove this
         getInstance().registerTurtleUpgradeModeller(serialiser, modeller);
     }
 

projects/common-api/src/client/java/dan200/computercraft/api/client/turtle/RegisterTurtleUpgradeModeller.java

@@ -0,0 +1,26 @@
+// SPDX-FileCopyrightText: 2023 The CC: Tweaked Developers
+//
+// SPDX-License-Identifier: MPL-2.0
+
+package dan200.computercraft.api.client.turtle;
+
+import dan200.computercraft.api.turtle.ITurtleUpgrade;
+import dan200.computercraft.api.turtle.TurtleUpgradeSerialiser;
+
+/**
+ * A functional interface to register a {@link TurtleUpgradeModeller} for a class of turtle upgrades.
+ * <p>
+ * This interface is largely intended to be used from multi-loader code, to allow sharing registration code between
+ * multiple loaders.
+ */
+@FunctionalInterface
+public interface RegisterTurtleUpgradeModeller {
+    /**
+     * Register a {@link TurtleUpgradeModeller}.
+     *
+     * @param serialiser The turtle upgrade serialiser.
+     * @param modeller   The upgrade modeller.
+     * @param <T>        The type of the turtle upgrade.
+     */
+    <T extends ITurtleUpgrade> void register(TurtleUpgradeSerialiser<T> serialiser, TurtleUpgradeModeller<T> modeller);
+}

projects/common-api/src/client/java/dan200/computercraft/api/client/turtle/TurtleUpgradeModeller.java

@@ -4,12 +4,10 @@
 
 package dan200.computercraft.api.client.turtle;
 
-import dan200.computercraft.api.client.ComputerCraftAPIClient;
 import dan200.computercraft.api.client.TransformedModel;
 import dan200.computercraft.api.turtle.ITurtleAccess;
 import dan200.computercraft.api.turtle.ITurtleUpgrade;
 import dan200.computercraft.api.turtle.TurtleSide;
-import dan200.computercraft.api.turtle.TurtleUpgradeSerialiser;
 import net.minecraft.client.resources.model.ModelResourceLocation;
 import net.minecraft.client.resources.model.UnbakedModel;
 import net.minecraft.nbt.CompoundTag;
@@ -21,9 +19,13 @@ import java.util.List;
 
 /**
  * Provides models for a {@link ITurtleUpgrade}.
+ * <p>
+ * Use {@code dan200.computercraft.api.client.FabricComputerCraftAPIClient#registerTurtleUpgradeModeller} to register a
+ * modeller on Fabric and {@code dan200.computercraft.api.client.turtle.RegisterTurtleModellersEvent} to register one
+ * on Forge
  *
  * @param <T> The type of turtle upgrade this modeller applies to.
- * @see ComputerCraftAPIClient#registerTurtleUpgradeModeller(TurtleUpgradeSerialiser, TurtleUpgradeModeller) To register a modeller.
+ * @see RegisterTurtleUpgradeModeller For multi-loader registration support.
  */
 public interface TurtleUpgradeModeller<T extends ITurtleUpgrade> {
     /**

projects/common-api/src/main/java/dan200/computercraft/api/turtle/TurtleUpgradeSerialiser.java

@@ -48,13 +48,8 @@ import java.util.function.Function;
  * }
  * }</pre>
  * <p>
- * Finally, we need to register a model for our upgrade. This is done with
- * {@link dan200.computercraft.api.client.ComputerCraftAPIClient#registerTurtleUpgradeModeller}:
- *
- * <pre>{@code
- * // Register our model inside FMLClientSetupEvent
- * ComputerCraftAPIClient.registerTurtleUpgradeModeller(MY_UPGRADE.get(), TurtleUpgradeModeller.flatItem())
- * }</pre>
+ * Finally, we need to register a model for our upgrade. The way to do this varies on mod loader, see
+ * {@link dan200.computercraft.api.client.turtle.TurtleUpgradeModeller} for more information.
  * <p>
  * {@link TurtleUpgradeDataProvider} provides a data provider to aid with generating these JSON files.
  *