Don't look at last modified date when crawling the source of a spec

[tidoust]

Via w3c/strudy#524 (comment)

The last modified date can only be trusted when the crawler crawls the generated (static) version of the spec. When it crawls the source of a spec, the cross-reference database and other external sources may have changed in the meantime. That changes the generated content without changing the source and thus does not change the last modified date. Reusing previous crawl results may be wrong as a result.

Such specs should probably always be crawled. It would be worth evaluating how many specs would be impacted though.

[dontcallmedom]

(presumably, this only affects link extraction - except when respec itself is updated, in which case it may have much broader impact)