Import error when debugging youtube-dl in VS Code

I'm trying to write a new extractor for youtube-dl. First I want to debug the __main__.py to get to know the tool, but I cannot debug using VS Code. Here's my launch.json:

{
    "version": "0.2.0",
    "configurations": [
        {
            "name": "Python: Current File",
            "type": "python",
            "request": "launch",
            "program": "${file}",
            "console": "integratedTerminal",
            "args": ["a_youtube_video"]
        }
    ]
}

My breakpoint is set in the __main__.py, which looks like this:

from __future__ import unicode_literals

# Execute with
# $ python youtube_dl/__main__.py (2.6+)
# $ python -m youtube_dl          (2.7+)

import sys

if __package__ is None and not hasattr(sys, 'frozen'):
    # direct call of __main__.py
    import os.path
    path = os.path.realpath(os.path.abspath(__file__))
    sys.path.insert(0, os.path.dirname(os.path.dirname(path)))

import youtube_dl

if __name__ == '__main__':
    youtube_dl.main() # Breakpoint here    

The error I'm facing is the import youtube_dl line, it reports that there's no module named youtube_dl. What am I missing here?

Edit: I've just found a way to debug it. It said right in the comments of the __main__.py: From 2.7, the program must be run as a module. However I still don't understand this module thing.