Allow users to constrain universal locks to specific markers

[charliermarsh]

I think we should let users apply a global marker constraint to "universal" resolution. E.g., maybe you don't care about locking on Windows, so you specify platform_system != 'Windows' in your pyproject.toml, and we avoid solving any branches that are Windows-only. Later, when installing, we'd error if the user is on an unsupported platform, based on those markers.

This would (1) make resolution a lot faster for users that only care about certain platforms, (2) enable us to solve locks that legitimately don't have solutions on certain platforms.

This would also solve #4087.

[zanieb]

I like it.

[BurntSushi]

I like this a lot. I feel like it's a good release valve.