Link module: validate internal links and ensure protocol-less URLs can pass validation

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.

Proposed solution

• find the internal path if it exists.
• for protocol-less URLS such as //domain.tld add an "https" or "http" depending on which the site is using (only for validation)