Begin to use the self-compiled Dagger within the :writing parts of the processor

This is meant to be a baby step, just adding the entry points of injected types. There's still plenty more that should be done to remove a lot of the manual DI.

RELNOTES=Dagger itself now uses more Dagger

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Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=226503266
diff --git a/java/dagger/internal/codegen/TopLevelImplementationComponent.java b/java/dagger/internal/codegen/TopLevelImplementationComponent.java
new file mode 100644
index 0000000..306c05d
--- /dev/null
+++ b/java/dagger/internal/codegen/TopLevelImplementationComponent.java
@@ -0,0 +1,41 @@
+/*
+ * Copyright (C) 2018 The Dagger Authors.
+ *
+ * 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 dagger.internal.codegen;
+
+import dagger.BindsInstance;
+import dagger.Module;
+import dagger.Subcomponent;
+
+/**
+ * A shared subcomponent for a top-level {@link ComponentImplementation} and any nested child
+ * implementations.
+ */
+@PerGeneratedFile
+@Subcomponent
+interface TopLevelImplementationComponent {
+  CurrentImplementationSubcomponent.Builder currentImplementationSubcomponentBuilder();
+
+  @Subcomponent.Builder
+  interface Builder {
+    @BindsInstance
+    Builder topLevelComponent(@TopLevel ComponentImplementation topLevelImplementation);
+    TopLevelImplementationComponent build();
+  }
+
+  @Module(subcomponents = TopLevelImplementationComponent.class)
+  interface InstallationModule {}
+}