Depend on config rule instead of file.

The aos_config rule used to require you to depend on {name}.json to get
the output.  That was weird and hid private/testonly/etc.  Instead,
depend on the rule directly.

Change-Id: I1596a6c1f44b246a0db1bbc562468530a0b69614
diff --git a/aos/BUILD b/aos/BUILD
index a7d691b..286882f 100644
--- a/aos/BUILD
+++ b/aos/BUILD
@@ -466,7 +466,7 @@
         "configuration_test.cc",
     ],
     data = [
-        "//aos/events:pingpong_config.json",
+        "//aos/events:pingpong_config",
         "//aos/events:pong.bfbs",
         "//aos/testdata:test_configs",
     ],