Move `PhutilBootloader` into the standard directory structure

Summary: Move `PhutilBootloader` and other related classes/functions into the standard directory structure. Whilst there is no compelling reason that we have to do this, it does make the code in `libphutil/src/__phutil_library_init__.php` a bit cleaner.

Test Plan: Verfied that `arc liberate` and `arc unit` still worked.

Reviewers: epriestley, #blessed_reviewers

Reviewed By: epriestley, #blessed_reviewers

Subscribers: epriestley, Korvin

Differential Revision: https://secure.phabricator.com/D9818