Consolidate URL parsing RegExps in parse-spec-url

[tidoust]

See https://github.com/w3c/browser-specs/pull/325#pullrequestreview-698096555.

• compute-shortname uses a similar list of regular expressions as those in parse-spec-url in particular. It would be good to merge the logic.
• fetch-groups does its own parsing of datatracker URLs, which could be moved to parse-spec-url (and shared with the compute-shortname logic)