Break out EppResourceInput subclasses

This frankenclass was starting to get a little cluttered, especially once I add the ChildEntityInput/Reader.
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Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=117266882

java/com/google/domain/registry/mapreduce/inputs/EppResourceKeyInput.java

@@ -0,0 +1,45 @@
+// Copyright 2016 Google Inc. All Rights Reserved.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+package com.google.domain.registry.mapreduce.inputs;
+
+import com.google.appengine.tools.mapreduce.Input;
+import com.google.appengine.tools.mapreduce.InputReader;
+import com.google.common.collect.ImmutableSet;
+import com.google.domain.registry.model.EppResource;
+import com.google.domain.registry.model.index.EppResourceIndexBucket;
+
+import com.googlecode.objectify.Key;
+
+/**
+ * A MapReduce {@link Input} that loads keys to all {@link EppResource} objects of a given type.
+ *
+ * <p>When mapping over keys we can't distinguish between Objectify polymorphic types.
+ */
+class EppResourceKeyInput<R extends EppResource> extends EppResourceBaseInput<Key<R>> {
+
+  private static final long serialVersionUID = -5426821384707653743L;
+
+  private final ImmutableSet<Class<? extends R>> resourceClasses;
+
+  public EppResourceKeyInput(ImmutableSet<Class<? extends R>> resourceClasses) {
+    this.resourceClasses = resourceClasses;
+    checkResourceClassesForInheritance(resourceClasses);
+  }
+
+  @Override
+  protected InputReader<Key<R>> bucketToReader(Key<EppResourceIndexBucket> bucketKey) {
+    return new EppResourceKeyReader<>(bucketKey, resourceClasses);
+  }
+}