completion/selectrum -> completion/vertico, part 2

- Rename module from `:completion selectrum` to `:completion vertico`
- Rename all files involved
- Do *not* yet rename all the functions, as that messes up git's rename
  detection.

modules/tools/lookup/autoload/lookup.el

@@ -245,7 +245,7 @@ Will return nil if neither is available. These require ripgrep to be installed."
               ((featurep! :completion helm)
                (+helm-file-search :query query)
                t)
-              ((featurep! :completion selectrum)
+              ((featurep! :completion vertico)
                (+selectrum-file-search :query query)
                t))))))