linkerror could prune pluralized link errors from being posted

[dlrobertson]

Summary

As part of whatwg/fetch#1580, it was noted that links for URL and URLs were used and were ambiguous. Errors for both were reported, but I think we only need to post this error once.

Minimal Example

❯ cat ./ambiguouslinks.bs
<pre class=metadata>
Title: Ambiguous Links
Group: test
Shortname: ambiguouslinks
Level: 1
Status: LS
Abstract: Testing too many ambiguous link errors
Editor: Example Editor
Date: 1970-01-01
</pre>

<!-- Would be nice if only one link error was posted -->
<a>URL</a>
<a>URLs</a>

❯ bikeshed spec ./ambiguouslinks.bs
LINK ERROR: Multiple possible 'url' dfn refs.
Arbitrarily chose https://w3c.github.io/webappsec-csp/#violation-url
To auto-select one of the following refs, insert one of these lines into a <pre class=link-defaults> block:
spec:csp3; type:dfn; text:url
spec:dom; type:dfn; text:url
spec:fetch; type:dfn; for:request; text:url
spec:fetch; type:dfn; for:response; text:url
spec:html; type:dfn; text:url
spec:reporting-1; type:dfn; text:url
spec:url; type:dfn; for:/; text:url
spec:url; type:dfn; for:basic URL parser; text:url
spec:webdriver-bidi; type:dfn; text:url
spec:prefetch; type:dfn; text:url
<a data-link-type="dfn" data-lt="URL">URL</a>
LINK ERROR: Multiple possible 'urls' dfn refs.
Arbitrarily chose https://w3c.github.io/webappsec-csp/#violation-url
To auto-select one of the following refs, insert one of these lines into a <pre class=link-defaults> block:
spec:csp3; type:dfn; text:url
spec:dom; type:dfn; text:url
spec:fetch; type:dfn; for:request; text:url
spec:fetch; type:dfn; for:response; text:url
spec:html; type:dfn; text:url
spec:reporting-1; type:dfn; text:url
spec:url; type:dfn; for:/; text:url
spec:url; type:dfn; for:basic URL parser; text:url
spec:webdriver-bidi; type:dfn; text:url
spec:prefetch; type:dfn; text:url
<a data-link-type="dfn" data-lt="URLs">URLs</a>
 ✔  Successfully generated, with 2 linking errors