Use phutil_var_export in PhutilLibraryMapBuilder

Summary: phutil_var_export produces slightly-more-readable output than var_export. Depends on D9923.

Test Plan: Regenerated src/__phutil_library_map__.php.

Reviewers: epriestley, Blessed Reviewers

Reviewed By: epriestley, Blessed Reviewers

Subscribers: epriestley, Korvin

Differential Revision: https://secure.phabricator.com/D9924