Feature request: ignore extras in constraints files, don't reject them

[ncoghlan]

What's the problem this feature will solve?

We have some CI pipelines that use the full locked set of project dependencies as a constraints file when installing a subset of the dependencies.

Rejecting extras in constraints files means that we can't do this with the new resolver, we have to pre-process the locked requirements file to remove all the extras declarations.

Describe the solution you'd like
Silently ignore extras in constraints files, don't fail the command.

Alternative Solutions

The workaround is to run sed -i 's/\[[^]]*\]//g' constraints.txt to pre-process a copy of the project requirements file to remove all the extras declarations, rather than using the requirements file directly.

Additional context

#6628 was an issue with the old resolver where extras declarations in constraints files were incorrectly interpreted as requesting that those extras be installed. This is not a request to bring back that behaviour - instead, it's a request to process constraints files as if they didn't contain any extras declarations, regardless of whether they do or not.

[ulope]

We're running into a similar problem.
In our project developers can place additional tools they like to have installed into a git excluded file which is installed as part of the dev install make target.
To ensure those additional tools don't cause conflicts we pass the our regular fully locked requirements-dev.txt file as a constraints file.
This no longer works with the new resolver as described by @ncoghlan.