Allowing '--extern' usage in build scripts and in config files

[notdanilo]

What does this PR try to resolve?

This PR implements #14065, allowing build scripts to automatically set dependencies, an important requirement for codegen tools.

[rustbot]

Thanks for the pull request, and welcome! The Rust team is excited to review your changes, and you should hear from @weihanglo (or someone else) some time within the next two weeks.

Please see the contribution instructions for more information. Namely, in order to ensure the minimum review times lag, PR authors and assigned reviewers should ensure that the review label (S-waiting-on-review and S-waiting-on-author) stays updated, invoking these commands when appropriate:

• @rustbot author: the review is finished, PR author should check the comments and take action accordingly
• @rustbot review: the author is ready for a review, this PR will be queued again in the reviewer's queue

[epage]

Thank you for your enthusiasm in wanting to improve cargo! However, our contribution guidelines say that Issues must be accepted by the cargo team before they will be reviewed

[weihanglo]

Thanks for the PR. Marked it as draft for now to reflect the fact that it's still in discussion.

[bors]

☔ The latest upstream changes (presumably #14193) made this pull request unmergeable. Please resolve the merge conflicts.