Simplify the use of phutil_get_library_root

Summary:
Currently, in order to retrieve the library root of the current phutil module, the following code is required:

$library = phutil_get_current_library_name();
$root = phutil_get_library_root($library);

This can be simplified by allowing the use of phutil_get_library_root() (without any arguments) to mean "get the library root for the current module".

Test Plan: N/A

Reviewers: #blessed_reviewers, epriestley

Reviewed By: #blessed_reviewers, epriestley

Subscribers: Korvin, epriestley

Differential Revision: https://secure.phabricator.com/D11326