R1:77ed7ade66f1

Add symbol import scripts for crossref features

Summary: Adds a script to import symbols from a ctags-like format, and another
to generate that format for PHP files.

Test Plan:
Ran it on Phabricator:

mysql> select * from repository_symbol limit 200, 20;

+-------------------+---------------------------------------------------+------------+----------------+--------+------------+
| arcanistProjectID | symbolName |
symbolType | symbolLanguage | pathID | lineNumber…