MenuLinkParent breaks migration when Parent URI is external

Problem/Motivation

Migrating from a drupal 6 site and get this error:

[error] The internal path component 'https://example.com/publications.html' is external. You are not allowed to specify an external URL together with internal:/.''

Which is coming from MenuLinkParent because of the assumption that the parent link path is routeable through $url = Url::fromUserInput("/$parent_link_path");

Proposed resolution

Check for external and search for the link uri properties.

Remaining tasks

User interface changes

API changes

Data model changes