Add support for --find-links in pip-compile input

[estan]

This adds support to pip-compile for the -f, --find-links <url> flag in the input requirements.in.

Let me know if there should be a corresponding --emit-find-links / --no-emit-find-links as I wasn't sure.

I also took the liberty of moving down the deduplication of repository.finder.index_urls/find_links so that it's after the last parse_requirements call, otherwise dupes creep in after the deduplication.

Changelog-friendly one-liner: Add support for --find-links in pip-compile input

@nvie @vphilippon @davidovich

[graingert]

@nvie any update on this?
@estan can you resolve the conflicts please?

[estan]

Sorry been completely swamped at work lately. Will try to resolve conflicts tomorrow when at the office.

[nvie]

@nvie any update on this?

Sorry, I'm not actively involved in this project anymore. I had too many projects going on, and had to let some go. That's why I transferred this project to the jazzband org, so others can keep maintaining it.

[atugushev]

Closed in favour of #793.