support namespace notation in extends clauses

Summary:

For now, just convert these to their string representation
as a single ident with a fake token. This will probably
require something more sophisticated.

Note: this adds a dependency on `lang_ml` to `lang_php`
to support printing Really Obvious error messages if
this still goes wrong.

Test Plan:

Generate code graph; doesn't raise `Todo`.

Reviewed By: pieter

CC:

Task ID: #

Blame Rev: