make calling_context more accurate

[blurrcat]

As detailed in https://github.com/mgood/flask-debugtoolbar/issues/96, the current logic for generating the calling context often fails unnecessarily.

Specifically, when a project is structured like the following,

project
├── project
│   ├── app.py
│   └── views.py
└── setup.py

and the flask application is defined by app = Flask(__name__) in app.py, the calling context will be "unknown" if the query is issued from somewhere other than app.py. This is because now app_path is project.app but the relevant frame that issues the query might be project.views.

The updated logic matches the frame with the top level package of the app_path.