Ignore yarn.lock and package-lock.json in oss

Summary:
Attn janicduplessis
Closes https://github.com/facebook/react-native/pull/18281

Differential Revision: D7200858

Pulled By: hramos

fbshipit-source-id: 8566251255a12aba3a3f9d6af532c4f842a8c41e

.gitignore

@@ -54,7 +54,8 @@ node_modules
 
 # TODO: Check in yarn.lock in open source. Right now we need to keep it out
 # from the GitHub repo as importing it might conflict with internal workspaces
-*/**/yarn.lock
+yarn.lock
+package-lock.json
 
 # OS X
 .DS_Store