Expose the file symbol map from PhutilLibraryMapBuilder

Summary:
Ref T5640. In D9864, collection of output from `phutil_symbols.php` (which includes symbols that things //use//) was replaced with processed output (which does not).

Allow callers (notably, `ArcanistPhutilLibraryLinter`) to access the raw have/need data.

Test Plan: See D9954.

Reviewers: joshuaspence

Reviewed By: joshuaspence

Subscribers: epriestley

Maniphest Tasks: T5640

Differential Revision: https://secure.phabricator.com/D9953