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