Move `PhutilLibraryMapBuilder` to `libphutil`

Summary: See D9813 for a detailed explanation.

Test Plan:
```
> ./bin/arc liberate
Finding source files...
Found 194 files.
Loading symbol cache...
Found 194 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/D9815