org-download.el: switch to 'url-parse.

- org-download-image-dir: Fix mismatch error (string set to nil.)
- org-download--backend-cmd: Removed.
- org-download-backend: Use a choice instead of a string reprensenting the tag.
  (org-download--fullname): Use url-parse to remove query params urls.
  Use `file-name-nondirectory' instead of splitting on slashes.
  (org-download--image): Use `org-download-backend' directly and
  generate better error messages for `url-retrieve' failures.