importd: validate local image names with the right helper

importd: validate local image names with the right helper

A while back we introduced image_name_is_valid() for validating image
file names. It's more liberal than hostname_is_valid() in many ways (and
allows version suffixes and such). Since importd deals in offline images
(as opposed to machined otherwise which deals in running machines),
let's hence use the right helper to validate the identifiers.