Ignore all files in "src/extensions/", not just immediate children
Summary: Fixes T13418. This test ignores files in any directory named "extensions/", but the intent is to ignore all descendants of "src/extensions/".
Test Plan: Put files in "src/extensions/xyz/QQQ.php", ran "arc liberate", got a clean library map without the class in "QQQ.php" indexed.
Maniphest Tasks: T13418
Differential Revision: https://secure.phabricator.com/D20824