ref(releases.conf): collapse source/owner/repo into single keys

Source type is now inferred from the primary key:
  github_repo = owner/repo   (was source=github + owner + repo)
  git_url = https://...      (was source=gittag + url)
  gitea_repo = owner/repo    (was source=gitea + owner + repo)
  hashicorp_product = name   (was source=hashicorp + product)

One-off dist sources (nodedist, zigdist, etc.) keep the explicit
source= key since they're already one-liners.

Parser still accepts the old format via the default fallback branch.

cmd/webicached/main.go

@@ -586,7 +586,7 @@ func fetchGPGDist(ctx context.Context, client *http.Client, rawDir, pkgName stri
 }
 
 func fetchHashiCorp(ctx context.Context, client *http.Client, rawDir, pkgName string, conf *installerconf.Conf) error {
-	product := conf.Extra["product"]
+	product := conf.Repo
 	if product == "" {
 		product = pkgName
 	}