Respect --find-links in requirements.in and put it in compiled requirements.txt

[wooyek]

I have had --find-links=requirements in requirements.txt. It point's to a folder containing proprietary libraries, this folder is deployed alongside the application to CI server, to Heroku, etc.

When I switched to requirements.in and pip-tools, my compiled requirements.txt does not contain it anymore. I know I can supply find-links switch in other ways in target environment, but leaving it in the requirements.txt file is the most simple and robust solution — no need to configure each and every environment separately.

It would be good to somehow whitelist or otherwise put --find-links=requirements in compiled `requirements.txt.

[suutari]

This is already implemented in PR #453, but it's still unmerged.

[wooyek]

Sweet :)