Improve the handling of the `php_compat_info.json` file.

Summary: See inline comments on D9576. Also replace `json_decode` with `phutil_json_decode`, for better error handling.

Test Plan: Make sure `arc unit` still works.

Reviewers: epriestley, #blessed_reviewers

Reviewed By: epriestley, #blessed_reviewers

Subscribers: epriestley, Korvin

Differential Revision: https://secure.phabricator.com/D9656