Add -c/--constraint option to pip-compile

[atugushev]

• resolves --constraint support #1364
• resolves Improve layered dependency workflow #1092
• resolves pip-compile --extra doesn't take into account already pinned versions #1891
• resolves Adding a nesting structure so that pip-compile can pin the same versions across nested extras. #1907
• https://stackoverflow.com/q/76055688/1529164.

Contributor checklist

• Provided the tests for the changes.
• Assure PR title is short, clear, and good to be included in the user-oriented changelog

Maintainer checklist

• Assure one of these labels is present: backwards incompatible, feature, enhancement, deprecation, bug, dependency, docs or skip-changelog as they determine changelog listing.
• Assign the PR to an existing or new milestone for the target version (following Semantic Versioning).

[AndydeCleyre]

All the other multiple=True options are using the exact phrase "may be used more than once" 🤷🏼

[atugushev]

Thanks! Addressed in d5e13f7.

[AndydeCleyre]

int?

[atugushev]

Thanks! Fixed in d5e13f7.

[atugushev]

Thanks for the review @AndydeCleyre 🙏🏻

[AndydeCleyre]

Not that it matters, but "not-an-list" -> "not-a-list"

[honnix]

Thank you so much for adding this command line option! It seems only local file is supported. Is that correct? pip, however can use constraints file from any URL: https://pip.pypa.io/en/stable/user_guide/#constraints-files. Maybe that could be supported as well by pip-compile?

[honnix]

Maybe it could be as simple as removing click.Path type? That would skip local file validation unfortunately.

Updated: I tried that locally and it seems working.

[honnix]

I proposed a change in #2038. We could go extra steps to validate local file and remote file, but it doesn't seem to be absolutely necessary.