Add --link-targets-dir argument to linkchecker

[pietroalbini]

In my release notes API list tool (#143053) I want to check whether all links generated by the tool are actually valid, and using linkchecker seems to be the most sensible choice.

Linkchecker currently has a fairly big limitation though: it can only check a single directory, it checks all of the files within it, and link targets must point inside that same directory. This works great when checking the whole documentation package, but in my case I only need to check that one file contains valid links to the standard library docs.

To solve that, this PR adds a new --link-targets-dir flag to linkchecker. Directories passed to it will be valid link targets (with lower priority than the root being checked), but links within them will not be checked.

I'm not that happy with the name of the flag, happy for it to be bikeshedded.

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[rustbot]

These commits modify the Cargo.lock file. Unintentional changes to Cargo.lock can be introduced when switching branches and rebasing PRs.

If this was unintentional then you should revert the changes before this PR is merged.
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