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