Automatically create PRs to tracks when fetch script(s) change

Whenever the `scripts/fetch-configlet` or `scripts/fetch-configlet.ps1` scripts change, we should automatically submit a PR to all the tracks that contain those file(s).

Small note: some tracks might have renamed the script to `fetch_configlet` or `fetch_configlet.sh` or `fetch-configlet.sh`. This is probably okay, as long as our automatic update script recognizes these alternative names.

Start by locating the automation that updates track repositories and inspect how it detects scripts and submits pull requests. Trace handling for scripts/fetch-configlet and scripts/fetch-configlet.ps1, then account for the listed alternative names. Done means changes to either source script produce PRs for every track containing a recognized copy.

Written by the indexing model from the issue text.