Read extension classes and functions from php_compat_info.json.

Summary: Consolidate php_extension_classes.txt and php_extension_functions.txt with php_compat_info.json. Given that php_extension_classes.txt and php_extension_functions.txt are manually generated whereas php_compat_info.json is generated automatically, this should make maintenance easier.

Test Plan: Deleted the src/.phutil_module_cache file (in each of rARC, rPHU and rP) and ran arc liberate.

Reviewers: Blessed Reviewers, epriestley

Reviewed By: Blessed Reviewers, epriestley

Subscribers: epriestley, Korvin

Differential Revision: https://secure.phabricator.com/D9662