v0.37.0 — opt-in imported-by badge for Go modules

create-badges gains `importers` (default false), rendering importers.svg plus
an importers.md backlink list of the importing repositories, grouped and flagged
by owner.

pkg.go.dev has no API for this, so the scrape self-checks: it reads the total
both from the links and from the count the page states, and refuses to render
anything if the two disagree -- on a schedule, a wrongly-rendered 0 would
overwrite a real count and still look like data.