Move GCP proxy code to the old [] proxy's location

1. Moved code for the GCP proxy to where the [] proxy code used to live.
3. Corrected reference to the GCP proxy location.
4. Misc changes to make ErrorProne and various tools happy.

+diekmann to LGTM terraform whitelist change.

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Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=213630560

javatests/google/registry/testing/BUILD

@@ -9,10 +9,6 @@ java_library(
     name = "testing",
     srcs = glob(
         ["*.java"],
-        exclude = [
-            "FakeProxyConnectionMetadataProvider.java",
-            "FakeQuotaServerClient.java",
-        ],
     ),
     resources = [
         "logging.properties",