.../BUILD: fix build file/module bugs (#18)

Change this pattern
    if foo [not] in native.existing_rules():
to
    if [not] native.existing_rule(foo):
for efficiency.

PiperOrigin-RevId: 241954724
diff --git a/cc/deps.bzl b/cc/deps.bzl
index 7134dd6..f4a6210 100644
--- a/cc/deps.bzl
+++ b/cc/deps.bzl
@@ -13,5 +13,5 @@
     )
 
 def _maybe(repo_rule, name, **kwargs):
-    if name not in native.existing_rules():
+    if not native.existing_rule(name):
         repo_rule(name = name, **kwargs)