Suppose in raw markdown we have:

```markdown
See [foo][barr].

[bar]: https://link-to-bar
```

The intent is to link the text "foo", but because of a typo it used the
label barr instead of bar. The result is that the rendered html won't
have a link at all. The text will be rendered literally as
`[foo][barr]`. Since there's no link (not even a broken one), a link
checker run over the rendered output won't spot the problem.

The mdbook-linkcheck plugin correctly detects this issue. However, it
doesn't understand links that go between our two mdbook projects since
the relative paths in the source don't match the relative paths in the
rendered output. Since checking whether the links are non-broken is
already covered by `bin/check_links` (using linklint), we can disable
checking the link destinations altogether.