Cleanup pkg-config.py
Cleanup pkg-config.py

While making a seperate change I noticed this script
could use a re-factor.

- Add --debug/-d flag to output extra information to stderr.
- Move main logic into main() function (makes script more easily testable).
- Add #! line to make script runnable.

Review URL: https://codereview.chromium.org/1577163002

Cr-Commit-Position: 972c6d2dc6dd5efdad1377c0d224e03eb8f276f7@{#368967}

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