Summary:
The `ArcanistXHPASTLinter` class is becoming quite bloated. This diff separates the class into one-class-per-rule, which makes everything much more modular. One downside to this decoupling is that code reuse between linter rules is much more difficult, although this only affects a very small number of linter rules.
There is still some further work that could be done here, but I defer this until a later diff:
- Rewrite `ArcanistPhutilXHPASTLinter` using `ArcanistXHPASTLinterRule`.
- Change the unit tests so that they are truly only testing a single linter rule.
- Maybe improve the way in which linter configuration options are handled.
- Make it easier to keep track of the linter rule IDs (see T6859).
Test Plan: `arc unit`
Reviewers: epriestley, #blessed_reviewers
Reviewed By: epriestley, #blessed_reviewers
Subscribers: johnny-bit, epriestley, Korvin
Differential Revision: https://secure.phabricator.com/D10541
