Description of the need
Spinning this out of https://github.com/backdrop/backdrop-issues/issues/4055 and https://github.com/backdrop/backdrop-issues/issues/4054. From d7 module:
- URLS are not validated http://drupal.org/node/2247261
- Protocol-less URLs should be rewritten for validation (only for validation): http://drupal.org/node/2889683
The second relies on the former changes.
- find the internal path if it exists.
- for protocol-less URLS such as
//domain.tldadd an "https" or "http" depending on which the site is using (only for validation)