Plugin initialization hook

I have already asked about augmenting AST with new nodes in the initialization hook for plugins in
https://github.com/python/mypy/issues/5910

I was able to figure it out with get_back_class_hook and dummy base class like here
https://github.com/mkurnikov/django-stubs/blob/master/django-stubs/conf/__init__.pyi#L7

There's another use case.
In Django there's an INSTALLED_APPS setting:

INSTALLED_APPS = ('app1', 'app2')

which specifies apps to use to retrieve model definitions for database schema (using models.py files).

Those could be used in the model definition code like this

from django.db import models

class User(models.Model):
    member = models.ForeignKey(to='app1.Model1')

In plugin, I can parse to= value, figure out the right type of a model, and set as return type of __get__ to have a proper value for member attribute.
For that I need to use lookup_fully_qualified
https://github.com/python/mypy/blob/master/mypy/semanal.py#L3453
and this requires module to be present in the self.modules list.

I need to create new MypyFile instances to populate that attr myself, but parse_file() method belongs to BuildManager class
https://github.com/python/mypy/blob/master/mypy/build.py#L648
and I wasn't able to find how to access it from the ClassDefContext.

ClassDefContext から始め、mypy/build.py の BuildManager.parse_file() とどのように関連しているかを追跡し、次に mypy/semanal.py の lookup_fully_qualified() と self.modules コレクションを調べます。プラグインが、設定された Django アプリモジュールをモデル参照の解決に利用可能にする、サポートされた方法を備えた時点で、この issue は完了です。