Move `PhutilLibraryMapBuilder` to `libphutil`

Summary: Move `scripts/phutil_rebuild_map.php` and the `PhutilLibraryMapBuilder` class from rARC to rPHU. The intention here is to create a stronger `ArcanistInfrastructureTestCase` acceptance test (and the corresponding acceptance tests for rP and rPHU) by regenerating the library map file (to a temporary output) and comparing it to the existing library map file. This will be able to catch the case in which a new class/function has been added to a library, but `arc liberate` has not been executed.

Test Plan:
```
> ./scripts/phutil_rebuild_map.php src
Finding source files...
Found 344 files.
Loading symbol cache...
Found 344 files in cache.
Building library map...
Writing map...
Done.
```

Reviewers: #blessed_reviewers, epriestley

Reviewed By: #blessed_reviewers, epriestley

Subscribers: epriestley, Korvin

Differential Revision: https://secure.phabricator.com/D9813