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 |

+-------------------+---------------------------------------------------+------------+----------------+--------+------------+

|                 1 | DifferentialDiffViewController                    |

class | php | 52 | 19 |

|                 1 | DifferentialInlineCommentEditController           |

class | php | 308 | 19 |

|                 1 | DifferentialInlineCommentPreviewController        |

class | php | 10543 | 19 |

|                 1 | DifferentialRevisionEditController                |

class | php | 10544 | 19 |

|                 1 | DifferentialRevisionListController                |

class | php | 10545 | 19 |

|                 1 | DifferentialRevisionViewController                |

class | php | 142 | 19 |

|                 1 | DifferentialSubscribeController                   |

class | php | 10546 | 19 |

|                 1 | DifferentialRevisionListData                      |

class | php | 58 | 19 |

|                 1 | DifferentialCommentEditor                         |

class | php | 39 | 19 |

|                 1 | DifferentialRevisionEditor                        |

class | php | 42 | 24 |

|                 1 | DifferentialFieldSpecificationIncompleteException |

class | php | 10547 | 19 |

|                 1 | DifferentialFieldDataNotAvailableException        |

class | php | 10548 | 19 |

|                 1 | DifferentialFieldParseException                   |

class | php | 10549 | 19 |

|                 1 | DifferentialFieldValidationException              |

class | php | 10550 | 19 |

|                 1 | DifferentialFieldSelector                         |

class | php | 10551 | 19 |

|                 1 | DifferentialDefaultFieldSelector                  |

class | php | 10552 | 19 |

|                 1 | DifferentialApplyPatchFieldSpecification          |

class | php | 10553 | 19 |

|                 1 | DifferentialArcanistProjectFieldSpecification     |

class | php | 10554 | 19 |

|                 1 | DifferentialAuthorFieldSpecification              |

class | php | 10555 | 19 |

|                 1 | DifferentialFieldSpecification                    |

class | php | 10556 | 35 |

+-------------------+---------------------------------------------------+------------+----------------+--------+------------+

20 rows in set (0.00 sec)

Reviewers: jungejason, nh, tuomaspelkonen, aran

Reviewed By: tuomaspelkonen

CC: aran, tuomaspelkonen

Differential Revision: 898