Stubgen skips "vendor" directory

Bug Report

Stubgen skips over a directory called vendor without any error message.

To Reproduce

Create a project with this structure

a
  __init__.py
  vendor
    __init__.py

2. Run stubgen a

3. All that gets generated is out/a/__init__.pyi

I would expect the output to include out/a/vendor/__init__.pyi

Running stubgen a/vendor also fails to generate out/a/vendor/__init__.pyi, but cd'ing into a and then running stubgen vendor creates a/out/vendor/__init__.pyi

Your Environment

• Mypy version used: 0.790
• Mypy command-line flags: Included in problem description
• Mypy configuration options from mypy.ini (and other config files): Not sure where my config file is, but I haven't modified it.
• Python version used: 3.7
• Operating system and version: Debian DNU/Linux 9 (stretch)

Inizia riproducendo il problema con la struttura del progetto a/vendor mostrata e il punto di ingresso di stubgen. Traccia il modo in cui stubgen a e stubgen a/vendor attraversano le directory, quindi verifica che l'output generato includa out/a/vendor/__init__.pyi senza modificare il comportamento della directory di lavoro.