Fix duplicate input class from object builder mixins (#611)

We Fix this by marking the old v0 mixins as `OldMixinTarget`

fabric-object-builders-v0/src/main/java/net/fabricmc/fabric/mixin/object/builder/OldMixinBlock.java

@@ -28,7 +28,7 @@ import net.fabricmc.fabric.api.event.registry.BlockConstructedCallback;
 
 @Mixin(Block.class)
 @Deprecated
-public class MixinBlock {
+public class OldMixinBlock {
 	@Inject(method = "<init>(Lnet/minecraft/block/AbstractBlock$Settings;)V", at = @At("RETURN"))
 	public void init(AbstractBlock.Settings builder, CallbackInfo info) {
 		BlockConstructedCallback.EVENT.invoker().building(builder, (Block) (Object) this);

fabric-object-builders-v0/src/main/java/net/fabricmc/fabric/mixin/object/builder/OldMixinItem.java

@@ -27,7 +27,7 @@ import net.fabricmc.fabric.api.event.registry.ItemConstructedCallback;
 
 @Mixin(Item.class)
 @Deprecated
-public class MixinItem {
+public class OldMixinItem {
 	@Inject(method = "<init>(Lnet/minecraft/item/Item$Settings;)V", at = @At("RETURN"))
 	public void init(Item.Settings builder, CallbackInfo info) {
 		ItemConstructedCallback.EVENT.invoker().building(builder, (Item) (Object) this);

fabric-object-builders-v0/src/main/resources/fabric-object-builders-v0.mixins.json

@@ -3,8 +3,8 @@
   "package": "net.fabricmc.fabric.mixin.object.builder",
   "compatibilityLevel": "JAVA_8",
   "mixins": [
-    "MixinBlock",
-    "MixinItem"
+    "OldMixinBlock",
+    "OldMixinItem"
   ],
   "injectors": {
     "defaultRequire": 1