Add mapper to import domains from RDE deposits

With some additional cleanup by Ben McIlwain.

-------------
Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=146695665

java/google/registry/rde/imports/RdeImportsModule.java

@@ -28,9 +28,11 @@ import javax.servlet.http.HttpServletRequest;
 @Module
 public final class RdeImportsModule {
 
+  static final String PATH = "path";
+
   @Provides
-  @Parameter("path")
+  @Parameter(PATH)
   static String providePath(HttpServletRequest req) {
-    return RequestParameters.extractRequiredParameter(req, "path");
+    return RequestParameters.extractRequiredParameter(req, PATH);
   }
 }