Minor change to ArcanistPhutilLibraryLinter

Summary: Ref T5577. Modify ArcanistPhutilLibraryLinter to use PhutilLibraryMapBuilder instead of using an ExecFuture to run phutil_rebuild_map.php.

Test Plan: Ran arc lint.

Reviewers: epriestley, Blessed Reviewers

Reviewed By: epriestley, Blessed Reviewers

Subscribers: epriestley, Korvin

Maniphest Tasks: T5577

Differential Revision: https://secure.phabricator.com/D9864