Allow libphutil autoload to coexist with other autoload

Summary: If we fail to autoload we currently throw, but this breaks in the
presence of other autoloaders. Detect if there are other autoloaders and allow
them a chance to autoload.

Test Plan: Used PHPExcel to export excel documents (PHPExcel registers a
separate autoloader).

Reviewers: btrahan, jungejason

Reviewed By: btrahan

CC: aran, epriestley

Maniphest Tasks: T911

Differential Revision: https://secure.phabricator.com/D1747