Remap w3c.github.io/csswg-drafts to drafts.csswg.org during browser-specs lookup

[queengooborg]

Within the last release, a pull request was merged to BCD that replaces links to drafts.csswg.org with w3c.github.io/csswg-drafts. This was done because drafts.csswg.org is almost always unavailable, leaving MDN readers (including and especially contributors) with no spec to refer to. The downtime issues have persisted for over a year, with no end in sight. Because of this, various developers have created their own mirrors with reliable build engines, including https://andreubotella.com/csswg-auto-build/, whose code was then used to create a new official mirror.

However, due to internal conflict with CSSWG members, the official mirror has not been added to browser-specs (w3c/browser-specs#704). As a result, any spec URLs using the new mirror will show "Unknown specification". This PR adds a fix for this by mapping the URLs back to the broken drafts.csswg.org URLs during the search through browser-specs.

This fixes #7173 and fixes mdn/content#20772.

[sideshowbarker]

Given w3c/browser-specs#704 (comment):

Browser-specs now lists https://w3c.github.io/csswg-drafts/ URLs for CSS specs under the nightly.alternateUrls.

…it seems like we could now instead update our browser-specs client/consumer code to check the nightly.alternateUrls value in addition to the nightly and nightly.url values?

[queengooborg]

It looks like the series.nightlyUrl property would still need to be remapped, unless we want to enforce URLs for specific spec levels only?

[sideshowbarker]

I think this PR is superseded by the solution in #7262, right?

What I mean is, there doesn’t seem to be any reason to prefer this PR over #7262 — because #7262 solves the underlying problem reported in #7173 and mdn/content#20772 through a straightforward lookup into the existing browser-specs data (which now includes the alternate w3c.github.io URLs for all CSS specs), and so without needing to do the remapping approach used in this PR.

Is that correct?