Give core registry libraries public visibility

This is so Donuts can create entirely separate Bazel repositories that
link against the Domain Registry codebase.
-------------
Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=116182448

java/com/google/domain/registry/model/BUILD

@@ -65,6 +65,7 @@ java_library(
         "*/*.java",
         "*/*/*.java",
     ]) + ["FieldExposerRegistry.java"] + FIELD_EXPOSERS,
+    visibility = ["//visibility:public"],
     deps = [
         ":field_exposer_registry",
         ":field_exposers",