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