Update symbol generation scripts

Summary:
Scripts now return scoped symbols -- in particular, PHP class constants, fields, and methods. ctags gives some for other languages.

(Turns out XHPAST doesn't support traits. But no one uses traits anyway so it's probably fine.)

I couldn't find a list of the context types ctags uses (class/struct/union/enum/maybe others?), so the context code just ignores that. Also, it uses a blacklist for the symbol type instead of a whitelist because there are a ton, they vary by language, and I didn't want to unintentionally exclude anything (P480).

Test Plan: Scrape symbols from arcanist and phabricator. Upload them to sandbox. Search for things.

Reviewers: epriestley

Reviewed By: epriestley

CC: nh, aran, Korvin

Maniphest Tasks: T1602

Differential Revision: https://secure.phabricator.com/D3202