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