Separate map building and updating in `PhutilLibraryMapBuilder`

Summary: Separate the building of the library map from the writing of the library map to file. This is a much nicer approach.

Test Plan: Ran `./scripts/phutil_rebuild_map.php src` and `./scripts/phutil_rebuild_map.php --show src`.

Reviewers: epriestley, #blessed_reviewers

Reviewed By: epriestley, #blessed_reviewers

Subscribers: epriestley, Korvin

Differential Revision: https://secure.phabricator.com/D9861